Merge pull request #11874 from Microsoft/emitHelper

Move emit helpers into related transformers
2025-11-18 17:21:48 +00:00 · 2016-11-16 10:37:35 -08:00
parent a5d9e9624d 9a1a605f7c
commit 3a95f9244b
19 changed files with 1548 additions and 1411 deletions
@@ -19632,7 +19632,7 @@ namespace ts {

        function isNameOfModuleOrEnumDeclaration(node: Identifier) {
            const parent = node.parent;
-            return isModuleOrEnumDeclaration(parent) && node === parent.name;
+            return parent && isModuleOrEnumDeclaration(parent) && node === parent.name;
        }

        // When resolved as an expression identifier, if the given node references an exported entity, return the declaration
@@ -571,7 +571,7 @@ namespace ts {
     */
    export function append<T>(to: T[] | undefined, value: T | undefined): T[] | undefined {
        if (value === undefined) return to;
-        if (to === undefined) to = [];
+        if (to === undefined) return [value];
        to.push(value);
        return to;
    }
@@ -592,6 +592,16 @@ namespace ts {
        return to;
    }

+    /**
+     * Stable sort of an array. Elements equal to each other maintain their relative position in the array.
+     */
+    export function stableSort<T>(array: T[], comparer: (x: T, y: T) => Comparison = compareValues) {
+        return array
+            .map((_, i) => i) // create array of indices
+            .sort((x, y) => comparer(array[x], array[y]) || compareValues(x, y)) // sort indices by value then position
+            .map(i => array[i]); // get sorted array
+    }
+
    export function rangeEquals<T>(array1: T[], array2: T[], pos: number, end: number) {
        while (pos < end) {
            if (array1[pos] !== array2[pos]) {
@@ -2099,6 +2109,17 @@ namespace ts {
    }

    /** Remove an item from an array, moving everything to its right one space left. */
+    export function orderedRemoveItem<T>(array: T[], item: T): boolean {
+        for (let i = 0; i < array.length; i++) {
+            if (array[i] === item) {
+                orderedRemoveItemAt(array, i);
+                return true;
+            }
+        }
+        return false;
+    }
+
+    /** Remove an item by index from an array, moving everything to its right one space left. */
    export function orderedRemoveItemAt<T>(array: T[], index: number): void {
        // This seems to be faster than either `array.splice(i, 1)` or `array.copyWithin(i, i+ 1)`.
        for (let i = index; i < array.length - 1; i++) {
@@ -20,188 +20,6 @@ namespace ts {
    export function emitFiles(resolver: EmitResolver, host: EmitHost, targetSourceFile: SourceFile, emitOnlyDtsFiles?: boolean): EmitResult {
        const delimiters = createDelimiterMap();
        const brackets = createBracketsMap();
-
-        // emit output for the __extends helper function
-        const extendsHelper = `
-var __extends = (this && this.__extends) || function (d, b) {
-    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
-    function __() { this.constructor = d; }
-    d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
-};`;
-
-        // Emit output for the __assign helper function.
-        // This is typically used for JSX spread attributes,
-        // and can be used for object literal spread properties.
-        const assignHelper = `
-var __assign = (this && this.__assign) || Object.assign || function(t) {
-    for (var s, i = 1, n = arguments.length; i < n; i++) {
-        s = arguments[i];
-        for (var p in s) if (Object.prototype.hasOwnProperty.call(s, p))
-            t[p] = s[p];
-        if (typeof Object.getOwnPropertySymbols === "function")
-            for (var i = 0, p = Object.getOwnPropertySymbols(s); i < p.length; i++)
-                t[p[i]] = s[p[i]];
-    }
-    return t;
-};`;
-
-        const restHelper = `
-var __rest = (this && this.__rest) || function (s, e) {
-    var t = {};
-    for (var p in s) if (Object.prototype.hasOwnProperty.call(s, p) && e.indexOf(p) < 0)
-        t[p] = s[p];
-    if (typeof Object.getOwnPropertySymbols === "function")
-        for (var i = 0, p = Object.getOwnPropertySymbols(s); i < p.length; i++) if (e.indexOf(p[i]) < 0)
-            t[p[i]] = s[p[i]];
-    return t;
-};`;
-
-        // emit output for the __decorate helper function
-        const decorateHelper = `
-var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
-    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
-    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
-    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
-    return c > 3 && r && Object.defineProperty(target, key, r), r;
-};`;
-
-        // emit output for the __metadata helper function
-        const metadataHelper = `
-var __metadata = (this && this.__metadata) || function (k, v) {
-    if (typeof Reflect === "object" && typeof Reflect.metadata === "function") return Reflect.metadata(k, v);
-};`;
-
-        // emit output for the __param helper function
-        const paramHelper = `
-var __param = (this && this.__param) || function (paramIndex, decorator) {
-    return function (target, key) { decorator(target, key, paramIndex); }
-};`;
-
-        // emit output for the __awaiter helper function
-        const awaiterHelper = `
-var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {
-    return new (P || (P = Promise))(function (resolve, reject) {
-        function fulfilled(value) { try { step(generator.next(value)); } catch (e) { reject(e); } }
-        function rejected(value) { try { step(generator["throw"](value)); } catch (e) { reject(e); } }
-        function step(result) { result.done ? resolve(result.value) : new P(function (resolve) { resolve(result.value); }).then(fulfilled, rejected); }
-        step((generator = generator.apply(thisArg, _arguments)).next());
-    });
-};`;
-
-        // The __generator helper is used by down-level transformations to emulate the runtime
-        // semantics of an ES2015 generator function. When called, this helper returns an
-        // object that implements the Iterator protocol, in that it has `next`, `return`, and
-        // `throw` methods that step through the generator when invoked.
-        //
-        // parameters:
-        //  thisArg  The value to use as the `this` binding for the transformed generator body.
-        //  body     A function that acts as the transformed generator body.
-        //
-        // variables:
-        //  _       Persistent state for the generator that is shared between the helper and the
-        //          generator body. The state object has the following members:
-        //            sent() - A method that returns or throws the current completion value.
-        //            label  - The next point at which to resume evaluation of the generator body.
-        //            trys   - A stack of protected regions (try/catch/finally blocks).
-        //            ops    - A stack of pending instructions when inside of a finally block.
-        //  f       A value indicating whether the generator is executing.
-        //  y       An iterator to delegate for a yield*.
-        //  t       A temporary variable that holds one of the following values (note that these
-        //          cases do not overlap):
-        //          - The completion value when resuming from a `yield` or `yield*`.
-        //          - The error value for a catch block.
-        //          - The current protected region (array of try/catch/finally/end labels).
-        //          - The verb (`next`, `throw`, or `return` method) to delegate to the expression
-        //            of a `yield*`.
-        //          - The result of evaluating the verb delegated to the expression of a `yield*`.
-        //
-        // functions:
-        //  verb(n)     Creates a bound callback to the `step` function for opcode `n`.
-        //  step(op)    Evaluates opcodes in a generator body until execution is suspended or
-        //              completed.
-        //
-        // The __generator helper understands a limited set of instructions:
-        //  0: next(value?)     - Start or resume the generator with the specified value.
-        //  1: throw(error)     - Resume the generator with an exception. If the generator is
-        //                        suspended inside of one or more protected regions, evaluates
-        //                        any intervening finally blocks between the current label and
-        //                        the nearest catch block or function boundary. If uncaught, the
-        //                        exception is thrown to the caller.
-        //  2: return(value?)   - Resume the generator as if with a return. If the generator is
-        //                        suspended inside of one or more protected regions, evaluates any
-        //                        intervening finally blocks.
-        //  3: break(label)     - Jump to the specified label. If the label is outside of the
-        //                        current protected region, evaluates any intervening finally
-        //                        blocks.
-        //  4: yield(value?)    - Yield execution to the caller with an optional value. When
-        //                        resumed, the generator will continue at the next label.
-        //  5: yield*(value)    - Delegates evaluation to the supplied iterator. When
-        //                        delegation completes, the generator will continue at the next
-        //                        label.
-        //  6: catch(error)     - Handles an exception thrown from within the generator body. If
-        //                        the current label is inside of one or more protected regions,
-        //                        evaluates any intervening finally blocks between the current
-        //                        label and the nearest catch block or function boundary. If
-        //                        uncaught, the exception is thrown to the caller.
-        //  7: endfinally       - Ends a finally block, resuming the last instruction prior to
-        //                        entering a finally block.
-        //
-        // For examples of how these are used, see the comments in ./transformers/generators.ts
-        const generatorHelper = `
-var __generator = (this && this.__generator) || function (thisArg, body) {
-    var _ = { label: 0, sent: function() { if (t[0] & 1) throw t[1]; return t[1]; }, trys: [], ops: [] }, f, y, t;
-    return { next: verb(0), "throw": verb(1), "return": verb(2) };
-    function verb(n) { return function (v) { return step([n, v]); }; }
-    function step(op) {
-        if (f) throw new TypeError("Generator is already executing.");
-        while (_) try {
-            if (f = 1, y && (t = y[op[0] & 2 ? "return" : op[0] ? "throw" : "next"]) && !(t = t.call(y, op[1])).done) return t;
-            if (y = 0, t) op = [0, t.value];
-            switch (op[0]) {
-                case 0: case 1: t = op; break;
-                case 4: _.label++; return { value: op[1], done: false };
-                case 5: _.label++; y = op[1]; op = [0]; continue;
-                case 7: op = _.ops.pop(); _.trys.pop(); continue;
-                default:
-                    if (!(t = _.trys, t = t.length > 0 && t[t.length - 1]) && (op[0] === 6 || op[0] === 2)) { _ = 0; continue; }
-                    if (op[0] === 3 && (!t || (op[1] > t[0] && op[1] < t[3]))) { _.label = op[1]; break; }
-                    if (op[0] === 6 && _.label < t[1]) { _.label = t[1]; t = op; break; }
-                    if (t && _.label < t[2]) { _.label = t[2]; _.ops.push(op); break; }
-                    if (t[2]) _.ops.pop();
-                    _.trys.pop(); continue;
-            }
-            op = body.call(thisArg, _);
-        } catch (e) { op = [6, e]; y = 0; } finally { f = t = 0; }
-        if (op[0] & 5) throw op[1]; return { value: op[0] ? op[1] : void 0, done: true };
-    }
-};`;
-
-        // emit output for the __export helper function
-        const exportStarHelper = `
-function __export(m) {
-    for (var p in m) if (!exports.hasOwnProperty(p)) exports[p] = m[p];
-}`;
-
-        // emit output for the UMD helper function.
-        const umdHelper = `
-(function (dependencies, factory) {
-    if (typeof module === 'object' && typeof module.exports === 'object') {
-        var v = factory(require, exports); if (v !== undefined) module.exports = v;
-    }
-    else if (typeof define === 'function' && define.amd) {
-        define(dependencies, factory);
-    }
-})`;
-
-        const superHelper = `
-const _super = name => super[name];`;
-
-        const advancedSuperHelper = `
-const _super = (function (geti, seti) {
-    const cache = Object.create(null);
-    return name => cache[name] || (cache[name] = { get value() { return geti(name); }, set value(v) { seti(name, v); } });
-})(name => super[name], (name, value) => super[name] = value);`;
-
        const compilerOptions = host.getCompilerOptions();
        const languageVersion = getEmitScriptTarget(compilerOptions);
        const moduleKind = getEmitModuleKind(compilerOptions);
@@ -238,12 +56,7 @@ const _super = (function (geti, seti) {
        let currentSourceFile: SourceFile;
        let currentText: string;
        let currentFileIdentifiers: Map<string>;
-        let extendsEmitted: boolean;
-        let assignEmitted: boolean;
-        let restEmitted: boolean;
-        let decorateEmitted: boolean;
-        let paramEmitted: boolean;
-        let awaiterEmitted: boolean;
+        let bundledHelpers: Map<boolean>;
        let isOwnFileEmit: boolean;
        let emitSkipped = false;

@@ -308,12 +121,13 @@ const _super = (function (geti, seti) {
            nodeIdToGeneratedName = [];
            autoGeneratedIdToGeneratedName = [];
            generatedNameSet = createMap<string>();
+            bundledHelpers = isBundledEmit ? createMap<boolean>() : undefined;
            isOwnFileEmit = !isBundledEmit;

            // Emit helpers from all the files
            if (isBundledEmit && moduleKind) {
                for (const sourceFile of sourceFiles) {
-                    emitEmitHelpers(sourceFile);
+                    emitHelpers(sourceFile, /*isBundle*/ true);
                }
            }

@@ -348,11 +162,6 @@ const _super = (function (geti, seti) {
            tempFlags = TempFlags.Auto;
            currentSourceFile = undefined;
            currentText = undefined;
-            extendsEmitted = false;
-            assignEmitted = false;
-            decorateEmitted = false;
-            paramEmitted = false;
-            awaiterEmitted = false;
            isOwnFileEmit = false;
        }

@@ -861,6 +670,8 @@ const _super = (function (geti, seti) {
                // Transformation nodes
                case SyntaxKind.PartiallyEmittedExpression:
                    return emitPartiallyEmittedExpression(<PartiallyEmittedExpression>node);
+                case SyntaxKind.RawExpression:
+                    return writeLines((<RawExpression>node).text);
            }
        }

@@ -898,12 +709,7 @@ const _super = (function (geti, seti) {
        //

        function emitIdentifier(node: Identifier) {
-            if (getEmitFlags(node) & EmitFlags.UMDDefine) {
-                writeLines(umdHelper);
-            }
-            else {
-                write(getTextOfNode(node, /*includeTrivia*/ false));
-            }
+            write(getTextOfNode(node, /*includeTrivia*/ false));
        }

        //
@@ -2207,93 +2013,39 @@ const _super = (function (geti, seti) {
            return statements.length;
        }

-        function emitHelpers(node: Node) {
-            const emitFlags = getEmitFlags(node);
-            let helpersEmitted = false;
-            if (emitFlags & EmitFlags.EmitEmitHelpers) {
-                helpersEmitted = emitEmitHelpers(currentSourceFile);
-            }
-
-            if (emitFlags & EmitFlags.EmitExportStar) {
-                writeLines(exportStarHelper);
-                helpersEmitted = true;
-            }
-
-            if (emitFlags & EmitFlags.EmitSuperHelper) {
-                writeLines(superHelper);
-                helpersEmitted = true;
-            }
-
-            if (emitFlags & EmitFlags.EmitAdvancedSuperHelper) {
-                writeLines(advancedSuperHelper);
-                helpersEmitted = true;
-            }
-
-            return helpersEmitted;
-        }
-
-        function emitEmitHelpers(node: SourceFile) {
-            // Only emit helpers if the user did not say otherwise.
-            if (compilerOptions.noEmitHelpers) {
-                return false;
-            }
-
-            // Don't emit helpers if we can import them.
-            if (compilerOptions.importHelpers
-                && (isExternalModule(node) || compilerOptions.isolatedModules)) {
-                return false;
-            }
+        function emitHelpers(node: Node, isBundle?: boolean) {
+            const sourceFile = isSourceFile(node) ? node : currentSourceFile;
+            const shouldSkip = compilerOptions.noEmitHelpers || (sourceFile && getExternalHelpersModuleName(sourceFile) !== undefined);
+            const shouldBundle = isSourceFile(node) && !isOwnFileEmit;

            let helpersEmitted = false;
+            const helpers = getEmitHelpers(node);
+            if (helpers) {
+                for (const helper of stableSort(helpers, compareEmitHelpers)) {
+                    if (!helper.scoped) {
+                        // Skip the helper if it can be skipped and the noEmitHelpers compiler
+                        // option is set, or if it can be imported and the importHelpers compiler
+                        // option is set.
+                        if (shouldSkip) continue;

-            // Only Emit __extends function when target ES5.
-            // For target ES6 and above, we can emit classDeclaration as is.
-            if ((languageVersion < ScriptTarget.ES2015) && (!extendsEmitted && node.flags & NodeFlags.HasClassExtends)) {
-                writeLines(extendsHelper);
-                extendsEmitted = true;
-                helpersEmitted = true;
-            }
+                        // Skip the helper if it can be bundled but hasn't already been emitted and we
+                        // are emitting a bundled module.
+                        if (shouldBundle) {
+                            if (bundledHelpers[helper.name]) {
+                                continue;
+                            }

-            if ((languageVersion < ScriptTarget.ESNext || currentSourceFile.scriptKind === ScriptKind.JSX || currentSourceFile.scriptKind === ScriptKind.TSX) &&
-                compilerOptions.jsx !== JsxEmit.Preserve &&
-                !assignEmitted &&
-                node.flags & NodeFlags.HasSpreadAttribute) {
-                writeLines(assignHelper);
-                assignEmitted = true;
-            }
+                            bundledHelpers[helper.name] = true;
+                        }
+                    }
+                    else if (isBundle) {
+                        // Skip the helper if it is scoped and we are emitting bundled helpers
+                        continue;
+                    }

-            if (languageVersion < ScriptTarget.ESNext && !restEmitted && node.flags & NodeFlags.HasRestAttribute) {
-                writeLines(restHelper);
-                restEmitted = true;
-            }
-
-            if (!decorateEmitted && node.flags & NodeFlags.HasDecorators) {
-                writeLines(decorateHelper);
-                if (compilerOptions.emitDecoratorMetadata) {
-                    writeLines(metadataHelper);
+                    writeLines(helper.text);
+                    helpersEmitted = true;
                }
-
-                decorateEmitted = true;
-                helpersEmitted = true;
-            }
-
-            if (!paramEmitted && node.flags & NodeFlags.HasParamDecorators) {
-                writeLines(paramHelper);
-                paramEmitted = true;
-                helpersEmitted = true;
-            }
-
-            // Only emit __awaiter function when target ES5/ES6.
-            // Only emit __generator function when target ES5.
-            // For target ES2017 and above, we can emit async/await as is.
-            if ((languageVersion < ScriptTarget.ES2017) && (!awaiterEmitted && node.flags & NodeFlags.HasAsyncFunctions)) {
-                writeLines(awaiterHelper);
-                if (languageVersion < ScriptTarget.ES2015) {
-                    writeLines(generatorHelper);
-                }
-
-                awaiterEmitted = true;
-                helpersEmitted = true;
            }

            if (helpersEmitted) {
@@ -2304,9 +2056,10 @@ const _super = (function (geti, seti) {
        }

        function writeLines(text: string): void {
-            const lines = text.split(/\r\n|\r|\n/g);
+            const lines = text.split(/\r\n?|\n/g);
+            const indentation = guessIndentation(lines);
            for (let i = 0; i < lines.length; i++) {
-                const line = lines[i];
+                const line = indentation ? lines[i].slice(indentation) : lines[i];
                if (line.length) {
                    if (i > 0) {
                        writeLine();
@@ -2316,6 +2069,21 @@ const _super = (function (geti, seti) {
            }
        }

+        function guessIndentation(lines: string[]) {
+            let indentation: number;
+            for (const line of lines) {
+                for (let i = 0; i < line.length && (indentation === undefined || i < indentation); i++) {
+                    if (!isWhiteSpace(line.charCodeAt(i))) {
+                        if (indentation === undefined || i < indentation) {
+                            indentation = i;
+                            break;
+                        }
+                    }
+                }
+            }
+            return indentation;
+        }
+
        //
        // Helpers
        //
@@ -1465,7 +1465,6 @@ namespace ts {
            if (node.resolvedTypeReferenceDirectiveNames !== undefined) updated.resolvedTypeReferenceDirectiveNames = node.resolvedTypeReferenceDirectiveNames;
            if (node.imports !== undefined) updated.imports = node.imports;
            if (node.moduleAugmentations !== undefined) updated.moduleAugmentations = node.moduleAugmentations;
-            if (node.externalHelpersModuleName !== undefined) updated.externalHelpersModuleName = node.externalHelpersModuleName;
            return updateNode(updated, node);
        }

@@ -1530,6 +1529,19 @@ namespace ts {
        return node;
    }

+    /**
+     * Creates a node that emits a string of raw text in an expression position. Raw text is never
+     * transformed, should be ES3 compliant, and should have the same precedence as
+     * PrimaryExpression.
+     *
+     * @param text The raw text of the node.
+     */
+    export function createRawExpression(text: string) {
+        const node = <RawExpression>createNode(SyntaxKind.RawExpression);
+        node.text = text;
+        return node;
+    }
+
    // Compound nodes

    export function createComma(left: Expression, right: Expression) {
@@ -1742,278 +1754,8 @@ namespace ts {

    // Helpers

-    export function createHelperName(externalHelpersModuleName: Identifier | undefined, name: string) {
-        return externalHelpersModuleName
-            ? createPropertyAccess(externalHelpersModuleName, name)
-            : createIdentifier(name);
-    }
-
-    export function createExtendsHelper(externalHelpersModuleName: Identifier | undefined, name: Identifier) {
-        return createCall(
-            createHelperName(externalHelpersModuleName, "__extends"),
-            /*typeArguments*/ undefined,
-            [
-                name,
-                createIdentifier("_super")
-            ]
-        );
-    }
-
-    export function createAssignHelper(externalHelpersModuleName: Identifier | undefined, attributesSegments: Expression[]) {
-        return createCall(
-            createHelperName(externalHelpersModuleName, "__assign"),
-            /*typeArguments*/ undefined,
-            attributesSegments
-        );
-    }
-
-    export function createParamHelper(externalHelpersModuleName: Identifier | undefined, expression: Expression, parameterOffset: number, location?: TextRange) {
-        return createCall(
-            createHelperName(externalHelpersModuleName, "__param"),
-            /*typeArguments*/ undefined,
-            [
-                createLiteral(parameterOffset),
-                expression
-            ],
-            location
-        );
-    }
-
-    export function createMetadataHelper(externalHelpersModuleName: Identifier | undefined, metadataKey: string, metadataValue: Expression) {
-        return createCall(
-            createHelperName(externalHelpersModuleName, "__metadata"),
-            /*typeArguments*/ undefined,
-            [
-                createLiteral(metadataKey),
-                metadataValue
-            ]
-        );
-    }
-
-    export function createDecorateHelper(externalHelpersModuleName: Identifier | undefined, decoratorExpressions: Expression[], target: Expression, memberName?: Expression, descriptor?: Expression, location?: TextRange) {
-        const argumentsArray: Expression[] = [];
-        argumentsArray.push(createArrayLiteral(decoratorExpressions, /*location*/ undefined, /*multiLine*/ true));
-        argumentsArray.push(target);
-        if (memberName) {
-            argumentsArray.push(memberName);
-            if (descriptor) {
-                argumentsArray.push(descriptor);
-            }
-        }
-
-        return createCall(createHelperName(externalHelpersModuleName, "__decorate"), /*typeArguments*/ undefined, argumentsArray, location);
-    }
-
-    export function createAwaiterHelper(externalHelpersModuleName: Identifier | undefined, hasLexicalArguments: boolean, promiseConstructor: EntityName | Expression, body: Block) {
-        const generatorFunc = createFunctionExpression(
-            /*modifiers*/ undefined,
-            createToken(SyntaxKind.AsteriskToken),
-            /*name*/ undefined,
-            /*typeParameters*/ undefined,
-            /*parameters*/ [],
-            /*type*/ undefined,
-            body
-        );
-
-        // Mark this node as originally an async function
-        (generatorFunc.emitNode || (generatorFunc.emitNode = {})).flags |= EmitFlags.AsyncFunctionBody;
-
-        return createCall(
-            createHelperName(externalHelpersModuleName, "__awaiter"),
-            /*typeArguments*/ undefined,
-            [
-                createThis(),
-                hasLexicalArguments ? createIdentifier("arguments") : createVoidZero(),
-                promiseConstructor ? createExpressionFromEntityName(promiseConstructor) : createVoidZero(),
-                generatorFunc
-            ]
-        );
-    }
-
-    export function createHasOwnProperty(target: LeftHandSideExpression, propertyName: Expression) {
-        return createCall(
-            createPropertyAccess(target, "hasOwnProperty"),
-            /*typeArguments*/ undefined,
-            [propertyName]
-        );
-    }
-
-    function createObjectCreate(prototype: Expression) {
-        return createCall(
-            createPropertyAccess(createIdentifier("Object"), "create"),
-            /*typeArguments*/ undefined,
-            [prototype]
-        );
-    }
-
-    function createGeti(target: LeftHandSideExpression) {
-        // name => super[name]
-        return createArrowFunction(
-            /*modifiers*/ undefined,
-            /*typeParameters*/ undefined,
-            [createParameter(/*decorators*/ undefined, /*modifiers*/ undefined, /*dotDotDotToken*/ undefined, "name")],
-            /*type*/ undefined,
-            createToken(SyntaxKind.EqualsGreaterThanToken),
-            createElementAccess(target, createIdentifier("name"))
-        );
-    }
-
-    function createSeti(target: LeftHandSideExpression) {
-        // (name, value) => super[name] = value
-        return createArrowFunction(
-            /*modifiers*/ undefined,
-            /*typeParameters*/ undefined,
-            [
-                createParameter(/*decorators*/ undefined, /*modifiers*/ undefined, /*dotDotDotToken*/ undefined, "name"),
-                createParameter(/*decorators*/ undefined, /*modifiers*/ undefined, /*dotDotDotToken*/ undefined, "value")
-            ],
-            /*type*/ undefined,
-            createToken(SyntaxKind.EqualsGreaterThanToken),
-            createAssignment(
-                createElementAccess(
-                    target,
-                    createIdentifier("name")
-                ),
-                createIdentifier("value")
-            )
-        );
-    }
-
-    export function createAdvancedAsyncSuperHelper() {
-        //  const _super = (function (geti, seti) {
-        //      const cache = Object.create(null);
-        //      return name => cache[name] || (cache[name] = { get value() { return geti(name); }, set value(v) { seti(name, v); } });
-        //  })(name => super[name], (name, value) => super[name] = value);
-
-        // const cache = Object.create(null);
-        const createCache = createVariableStatement(
-            /*modifiers*/ undefined,
-            createConstDeclarationList([
-                createVariableDeclaration(
-                    "cache",
-                    /*type*/ undefined,
-                    createObjectCreate(createNull())
-                )
-            ])
-        );
-
-        // get value() { return geti(name); }
-        const getter = createGetAccessor(
-            /*decorators*/ undefined,
-            /*modifiers*/ undefined,
-            "value",
-            /*parameters*/ [],
-            /*type*/ undefined,
-            createBlock([
-                createReturn(
-                    createCall(
-                        createIdentifier("geti"),
-                        /*typeArguments*/ undefined,
-                        [createIdentifier("name")]
-                    )
-                )
-            ])
-        );
-
-        // set value(v) { seti(name, v); }
-        const setter = createSetAccessor(
-            /*decorators*/ undefined,
-            /*modifiers*/ undefined,
-            "value",
-            [createParameter(/*decorators*/ undefined, /*modifiers*/ undefined, /*dotDotDotToken*/ undefined, "v")],
-            createBlock([
-                createStatement(
-                    createCall(
-                        createIdentifier("seti"),
-                        /*typeArguments*/ undefined,
-                        [
-                            createIdentifier("name"),
-                            createIdentifier("v")
-                        ]
-                    )
-                )
-            ])
-        );
-
-        // return name => cache[name] || ...
-        const getOrCreateAccessorsForName = createReturn(
-            createArrowFunction(
-                /*modifiers*/ undefined,
-                /*typeParameters*/ undefined,
-                [createParameter(/*decorators*/ undefined, /*modifiers*/ undefined, /*dotDotDotToken*/ undefined, "name")],
-                /*type*/ undefined,
-                createToken(SyntaxKind.EqualsGreaterThanToken),
-                createLogicalOr(
-                    createElementAccess(
-                        createIdentifier("cache"),
-                        createIdentifier("name")
-                    ),
-                    createParen(
-                        createAssignment(
-                            createElementAccess(
-                                createIdentifier("cache"),
-                                createIdentifier("name")
-                            ),
-                            createObjectLiteral([
-                                getter,
-                                setter
-                            ])
-                        )
-                    )
-                )
-            )
-        );
-
-        //  const _super = (function (geti, seti) {
-        //      const cache = Object.create(null);
-        //      return name => cache[name] || (cache[name] = { get value() { return geti(name); }, set value(v) { seti(name, v); } });
-        //  })(name => super[name], (name, value) => super[name] = value);
-        return createVariableStatement(
-            /*modifiers*/ undefined,
-            createConstDeclarationList([
-                createVariableDeclaration(
-                    "_super",
-                    /*type*/ undefined,
-                    createCall(
-                        createParen(
-                            createFunctionExpression(
-                                /*modifiers*/ undefined,
-                                /*asteriskToken*/ undefined,
-                                /*name*/ undefined,
-                                /*typeParameters*/ undefined,
-                                [
-                                    createParameter(/*decorators*/ undefined, /*modifiers*/ undefined, /*dotDotDotToken*/ undefined, "geti"),
-                                    createParameter(/*decorators*/ undefined, /*modifiers*/ undefined, /*dotDotDotToken*/ undefined, "seti")
-                                ],
-                                /*type*/ undefined,
-                                createBlock([
-                                    createCache,
-                                    getOrCreateAccessorsForName
-                                ])
-                            )
-                        ),
-                        /*typeArguments*/ undefined,
-                        [
-                            createGeti(createSuper()),
-                            createSeti(createSuper())
-                        ]
-                    )
-                )
-            ])
-        );
-    }
-
-    export function createSimpleAsyncSuperHelper() {
-        return createVariableStatement(
-            /*modifiers*/ undefined,
-            createConstDeclarationList([
-                createVariableDeclaration(
-                    "_super",
-                    /*type*/ undefined,
-                    createGeti(createSuper())
-                )
-            ])
-        );
+    export function getHelperName(name: string) {
+        return setEmitFlags(createIdentifier(name), EmitFlags.HelperName | EmitFlags.AdviseOnEmitNode);
    }

    // Utilities
@@ -2364,10 +2106,7 @@ namespace ts {
    }

    export function convertToFunctionBody(node: ConciseBody, multiLine?: boolean) {
-        if (isBlock(node)) {
-            return node;
-        }
-        return createBlock([createReturn(node, node)], node, multiLine);
+        return isBlock(node) ? node : createBlock([createReturn(node, /*location*/ node)], /*location*/ node, multiLine);
    }

    function isUseStrictPrologue(node: ExpressionStatement): boolean {
@@ -2419,14 +2158,21 @@ namespace ts {
        return statementOffset;
    }

+    export function startsWithUseStrict(statements: Statement[]) {
+        const firstStatement = firstOrUndefined(statements);
+        return firstStatement !== undefined
+            && isPrologueDirective(firstStatement)
+            && isUseStrictPrologue(firstStatement);
+    }
+
    /**
     * Ensures "use strict" directive is added
     *
-     * @param node source file
+     * @param statements An array of statements
     */
-    export function ensureUseStrict(node: SourceFile): SourceFile {
+    export function ensureUseStrict(statements: NodeArray<Statement>): NodeArray<Statement> {
        let foundUseStrict = false;
-        for (const statement of node.statements) {
+        for (const statement of statements) {
            if (isPrologueDirective(statement)) {
                if (isUseStrictPrologue(statement as ExpressionStatement)) {
                    foundUseStrict = true;
@@ -2437,13 +2183,15 @@ namespace ts {
                break;
            }
        }
+
        if (!foundUseStrict) {
-            const statements: Statement[] = [];
-            statements.push(startOnNewLine(createStatement(createLiteral("use strict"))));
-            // add "use strict" as the first statement
-            return updateSourceFileNode(node, statements.concat(node.statements));
+            return createNodeArray<Statement>([
+                startOnNewLine(createStatement(createLiteral("use strict"))),
+                ...statements
+            ], statements);
        }
-        return node;
+
+        return statements;
    }

    /**
@@ -2872,12 +2620,21 @@ namespace ts {
    }

    function mergeEmitNode(sourceEmitNode: EmitNode, destEmitNode: EmitNode) {
-        const { flags, commentRange, sourceMapRange, tokenSourceMapRanges } = sourceEmitNode;
-        if (!destEmitNode && (flags || commentRange || sourceMapRange || tokenSourceMapRanges)) destEmitNode = {};
+        const {
+            flags,
+            commentRange,
+            sourceMapRange,
+            tokenSourceMapRanges,
+            constantValue,
+            helpers
+        } = sourceEmitNode;
+        if (!destEmitNode) destEmitNode = {};
        if (flags) destEmitNode.flags = flags;
        if (commentRange) destEmitNode.commentRange = commentRange;
        if (sourceMapRange) destEmitNode.sourceMapRange = sourceMapRange;
        if (tokenSourceMapRanges) destEmitNode.tokenSourceMapRanges = mergeTokenSourceMapRanges(tokenSourceMapRanges, destEmitNode.tokenSourceMapRanges);
+        if (constantValue !== undefined) destEmitNode.constantValue = constantValue;
+        if (helpers) destEmitNode.helpers = addRange(destEmitNode.helpers, helpers);
        return destEmitNode;
    }

@@ -2914,7 +2671,7 @@ namespace ts {
     *
     * @param node The node.
     */
-    function getOrCreateEmitNode(node: Node) {
+    export function getOrCreateEmitNode(node: Node) {
        if (!node.emitNode) {
            if (isParseTreeNode(node)) {
                // To avoid holding onto transformation artifacts, we keep track of any
@@ -2955,6 +2712,16 @@ namespace ts {
        return node;
    }

+    /**
+     * Gets a custom text range to use when emitting source maps.
+     *
+     * @param node The node.
+     */
+    export function getSourceMapRange(node: Node) {
+        const emitNode = node.emitNode;
+        return (emitNode && emitNode.sourceMapRange) || node;
+    }
+
    /**
     * Sets a custom text range to use when emitting source maps.
     *
@@ -2966,6 +2733,18 @@ namespace ts {
        return node;
    }

+    /**
+     * Gets the TextRange to use for source maps for a token of a node.
+     *
+     * @param node The node.
+     * @param token The token.
+     */
+    export function getTokenSourceMapRange(node: Node, token: SyntaxKind) {
+        const emitNode = node.emitNode;
+        const tokenSourceMapRanges = emitNode && emitNode.tokenSourceMapRanges;
+        return tokenSourceMapRanges && tokenSourceMapRanges[token];
+    }
+
    /**
     * Sets the TextRange to use for source maps for a token of a node.
     *
@@ -2980,14 +2759,6 @@ namespace ts {
        return node;
    }

-    /**
-     * Sets a custom text range to use when emitting comments.
-     */
-    export function setCommentRange<T extends Node>(node: T, range: TextRange) {
-        getOrCreateEmitNode(node).commentRange = range;
-        return node;
-    }
-
    /**
     * Gets a custom text range to use when emitting comments.
     *
@@ -2999,25 +2770,11 @@ namespace ts {
    }

    /**
-     * Gets a custom text range to use when emitting source maps.
-     *
-     * @param node The node.
+     * Sets a custom text range to use when emitting comments.
     */
-    export function getSourceMapRange(node: Node) {
-        const emitNode = node.emitNode;
-        return (emitNode && emitNode.sourceMapRange) || node;
-    }
-
-    /**
-     * Gets the TextRange to use for source maps for a token of a node.
-     *
-     * @param node The node.
-     * @param token The token.
-     */
-    export function getTokenSourceMapRange(node: Node, token: SyntaxKind) {
-        const emitNode = node.emitNode;
-        const tokenSourceMapRanges = emitNode && emitNode.tokenSourceMapRanges;
-        return tokenSourceMapRanges && tokenSourceMapRanges[token];
+    export function setCommentRange<T extends Node>(node: T, range: TextRange) {
+        getOrCreateEmitNode(node).commentRange = range;
+        return node;
    }

    /**
@@ -3037,6 +2794,113 @@ namespace ts {
        return node;
    }

+    export function getExternalHelpersModuleName(node: SourceFile) {
+        const parseNode = getOriginalNode(node, isSourceFile);
+        const emitNode = parseNode && parseNode.emitNode;
+        return emitNode && emitNode.externalHelpersModuleName;
+    }
+
+    export function getOrCreateExternalHelpersModuleNameIfNeeded(node: SourceFile, compilerOptions: CompilerOptions) {
+        if (compilerOptions.importHelpers && (isExternalModule(node) || compilerOptions.isolatedModules)) {
+            const externalHelpersModuleName = getExternalHelpersModuleName(node);
+            if (externalHelpersModuleName) {
+                return externalHelpersModuleName;
+            }
+
+            const helpers = getEmitHelpers(node);
+            if (helpers) {
+                for (const helper of helpers) {
+                    if (!helper.scoped) {
+                        const parseNode = getOriginalNode(node, isSourceFile);
+                        const emitNode = getOrCreateEmitNode(parseNode);
+                        return emitNode.externalHelpersModuleName || (emitNode.externalHelpersModuleName = createUniqueName(externalHelpersModuleNameText));
+                    }
+                }
+            }
+        }
+    }
+    /**
+     * Adds an EmitHelper to a node.
+     */
+    export function addEmitHelper<T extends Node>(node: T, helper: EmitHelper): T {
+        const emitNode = getOrCreateEmitNode(node);
+        emitNode.helpers = append(emitNode.helpers, helper);
+        return node;
+    }
+
+    /**
+     * Adds an EmitHelper to a node.
+     */
+    export function addEmitHelpers<T extends Node>(node: T, helpers: EmitHelper[] | undefined): T {
+        if (some(helpers)) {
+            const emitNode = getOrCreateEmitNode(node);
+            for (const helper of helpers) {
+                if (!contains(emitNode.helpers, helper)) {
+                    emitNode.helpers = append(emitNode.helpers, helper);
+                }
+            }
+        }
+        return node;
+    }
+
+    /**
+     * Removes an EmitHelper from a node.
+     */
+    export function removeEmitHelper(node: Node, helper: EmitHelper): boolean {
+        const emitNode = node.emitNode;
+        if (emitNode) {
+            const helpers = emitNode.helpers;
+            if (helpers) {
+                return orderedRemoveItem(helpers, helper);
+            }
+        }
+        return false;
+    }
+
+    /**
+     * Gets the EmitHelpers of a node.
+     */
+    export function getEmitHelpers(node: Node): EmitHelper[] | undefined {
+        const emitNode = node.emitNode;
+        return emitNode && emitNode.helpers;
+    }
+
+    /**
+     * Moves matching emit helpers from a source node to a target node.
+     */
+    export function moveEmitHelpers(source: Node, target: Node, predicate: (helper: EmitHelper) => boolean) {
+        const sourceEmitNode = source.emitNode;
+        const sourceEmitHelpers = sourceEmitNode && sourceEmitNode.helpers;
+        if (!some(sourceEmitHelpers)) return;
+
+        const targetEmitNode = getOrCreateEmitNode(target);
+        let helpersRemoved = 0;
+        for (let i = 0; i < sourceEmitHelpers.length; i++) {
+            const helper = sourceEmitHelpers[i];
+            if (predicate(helper)) {
+                helpersRemoved++;
+                if (!contains(targetEmitNode.helpers, helper)) {
+                    targetEmitNode.helpers = append(targetEmitNode.helpers, helper);
+                }
+            }
+            else if (helpersRemoved > 0) {
+                sourceEmitHelpers[i - helpersRemoved] = helper;
+            }
+        }
+
+        if (helpersRemoved > 0) {
+            sourceEmitHelpers.length -= helpersRemoved;
+        }
+    }
+
+    export function compareEmitHelpers(x: EmitHelper, y: EmitHelper) {
+        if (x === y) return Comparison.EqualTo;
+        if (x.priority === y.priority) return Comparison.EqualTo;
+        if (x.priority === undefined) return Comparison.GreaterThan;
+        if (y.priority === undefined) return Comparison.LessThan;
+        return compareValues(x.priority, y.priority);
+    }
+
    export function setTextRange<T extends TextRange>(node: T, location: TextRange): T {
        if (location) {
            node.pos = location.pos;
@@ -3400,4 +3264,165 @@ namespace ts {
        Debug.assertNode(node, isExpression);
        return <Expression>node;
    }
+
+    export interface ExternalModuleInfo {
+        externalImports: (ImportDeclaration | ImportEqualsDeclaration | ExportDeclaration)[]; // imports of other external modules
+        externalHelpersImportDeclaration: ImportDeclaration | undefined; // import of external helpers
+        exportSpecifiers: Map<ExportSpecifier[]>; // export specifiers by name
+        exportedBindings: Map<Identifier[]>; // exported names of local declarations
+        exportedNames: Identifier[]; // all exported names local to module
+        exportEquals: ExportAssignment | undefined; // an export= declaration if one was present
+        hasExportStarsToExportValues: boolean; // whether this module contains export*
+    }
+
+    export function collectExternalModuleInfo(sourceFile: SourceFile, resolver: EmitResolver, compilerOptions: CompilerOptions): ExternalModuleInfo {
+        const externalImports: (ImportDeclaration | ImportEqualsDeclaration | ExportDeclaration)[] = [];
+        const exportSpecifiers = createMap<ExportSpecifier[]>();
+        const exportedBindings = createMap<Identifier[]>();
+        const uniqueExports = createMap<boolean>();
+        let exportedNames: Identifier[];
+        let hasExportDefault = false;
+        let exportEquals: ExportAssignment = undefined;
+        let hasExportStarsToExportValues = false;
+
+        const externalHelpersModuleName = getOrCreateExternalHelpersModuleNameIfNeeded(sourceFile, compilerOptions);
+        const externalHelpersImportDeclaration = externalHelpersModuleName && createImportDeclaration(
+            /*decorators*/ undefined,
+            /*modifiers*/ undefined,
+            createImportClause(/*name*/ undefined, createNamespaceImport(externalHelpersModuleName)),
+            createLiteral(externalHelpersModuleNameText));
+
+        if (externalHelpersImportDeclaration) {
+            externalImports.push(externalHelpersImportDeclaration);
+        }
+
+        for (const node of sourceFile.statements) {
+            switch (node.kind) {
+                case SyntaxKind.ImportDeclaration:
+                    // import "mod"
+                    // import x from "mod"
+                    // import * as x from "mod"
+                    // import { x, y } from "mod"
+                    externalImports.push(<ImportDeclaration>node);
+                    break;
+
+                case SyntaxKind.ImportEqualsDeclaration:
+                    if ((<ImportEqualsDeclaration>node).moduleReference.kind === SyntaxKind.ExternalModuleReference) {
+                        // import x = require("mod")
+                        externalImports.push(<ImportEqualsDeclaration>node);
+                    }
+
+                    break;
+
+                case SyntaxKind.ExportDeclaration:
+                    if ((<ExportDeclaration>node).moduleSpecifier) {
+                        if (!(<ExportDeclaration>node).exportClause) {
+                            // export * from "mod"
+                            externalImports.push(<ExportDeclaration>node);
+                            hasExportStarsToExportValues = true;
+                        }
+                        else {
+                            // export { x, y } from "mod"
+                            externalImports.push(<ExportDeclaration>node);
+                        }
+                    }
+                    else {
+                        // export { x, y }
+                        for (const specifier of (<ExportDeclaration>node).exportClause.elements) {
+                            if (!uniqueExports[specifier.name.text]) {
+                                const name = specifier.propertyName || specifier.name;
+                                multiMapAdd(exportSpecifiers, name.text, specifier);
+
+                                const decl = resolver.getReferencedImportDeclaration(name)
+                                    || resolver.getReferencedValueDeclaration(name);
+
+                                if (decl) {
+                                    multiMapAdd(exportedBindings, getOriginalNodeId(decl), specifier.name);
+                                }
+
+                                uniqueExports[specifier.name.text] = true;
+                                exportedNames = append(exportedNames, specifier.name);
+                            }
+                        }
+                    }
+                    break;
+
+                case SyntaxKind.ExportAssignment:
+                    if ((<ExportAssignment>node).isExportEquals && !exportEquals) {
+                        // export = x
+                        exportEquals = <ExportAssignment>node;
+                    }
+                    break;
+
+                case SyntaxKind.VariableStatement:
+                    if (hasModifier(node, ModifierFlags.Export)) {
+                        for (const decl of (<VariableStatement>node).declarationList.declarations) {
+                            exportedNames = collectExportedVariableInfo(decl, uniqueExports, exportedNames);
+                        }
+                    }
+                    break;
+
+                case SyntaxKind.FunctionDeclaration:
+                    if (hasModifier(node, ModifierFlags.Export)) {
+                        if (hasModifier(node, ModifierFlags.Default)) {
+                            // export default function() { }
+                            if (!hasExportDefault) {
+                                multiMapAdd(exportedBindings, getOriginalNodeId(node), getDeclarationName(<FunctionDeclaration>node));
+                                hasExportDefault = true;
+                            }
+                        }
+                        else {
+                            // export function x() { }
+                            const name = (<FunctionDeclaration>node).name;
+                            if (!uniqueExports[name.text]) {
+                                multiMapAdd(exportedBindings, getOriginalNodeId(node), name);
+                                uniqueExports[name.text] = true;
+                                exportedNames = append(exportedNames, name);
+                            }
+                        }
+                    }
+                    break;
+
+                case SyntaxKind.ClassDeclaration:
+                    if (hasModifier(node, ModifierFlags.Export)) {
+                        if (hasModifier(node, ModifierFlags.Default)) {
+                            // export default class { }
+                            if (!hasExportDefault) {
+                                multiMapAdd(exportedBindings, getOriginalNodeId(node), getDeclarationName(<ClassDeclaration>node));
+                                hasExportDefault = true;
+                            }
+                        }
+                        else {
+                            // export class x { }
+                            const name = (<ClassDeclaration>node).name;
+                            if (!uniqueExports[name.text]) {
+                                multiMapAdd(exportedBindings, getOriginalNodeId(node), name);
+                                uniqueExports[name.text] = true;
+                                exportedNames = append(exportedNames, name);
+                            }
+                        }
+                    }
+                    break;
+            }
+        }
+
+        return { externalImports, exportSpecifiers, exportEquals, hasExportStarsToExportValues, exportedBindings, exportedNames, externalHelpersImportDeclaration };
+    }
+
+    function collectExportedVariableInfo(decl: VariableDeclaration | BindingElement, uniqueExports: Map<boolean>, exportedNames: Identifier[]) {
+        if (isBindingPattern(decl.name)) {
+            for (const element of decl.name.elements) {
+                if (!isOmittedExpression(element)) {
+                    exportedNames = collectExportedVariableInfo(element, uniqueExports, exportedNames);
+                }
+            }
+        }
+        else if (!isGeneratedIdentifier(decl.name)) {
+            if (!uniqueExports[decl.name.text]) {
+                uniqueExports[decl.name.text] = true;
+                exportedNames = append(exportedNames, decl.name);
+            }
+        }
+        return exportedNames;
+    }
 }
@@ -27,84 +27,6 @@ namespace ts {
        EmitNotifications = 1 << 1,
    }

-    export interface TransformationResult {
-        /**
-         * Gets the transformed source files.
-         */
-        transformed: SourceFile[];
-
-        /**
-         * Emits the substitute for a node, if one is available; otherwise, emits the node.
-         *
-         * @param emitContext The current emit context.
-         * @param node The node to substitute.
-         * @param emitCallback A callback used to emit the node or its substitute.
-         */
-        emitNodeWithSubstitution(emitContext: EmitContext, node: Node, emitCallback: (emitContext: EmitContext, node: Node) => void): void;
-
-        /**
-         * Emits a node with possible notification.
-         *
-         * @param emitContext The current emit context.
-         * @param node The node to emit.
-         * @param emitCallback A callback used to emit the node.
-         */
-        emitNodeWithNotification(emitContext: EmitContext, node: Node, emitCallback: (emitContext: EmitContext, node: Node) => void): void;
-    }
-
-    export interface TransformationContext extends LexicalEnvironment {
-        getCompilerOptions(): CompilerOptions;
-        getEmitResolver(): EmitResolver;
-        getEmitHost(): EmitHost;
-
-        /**
-         * Hoists a function declaration to the containing scope.
-         */
-        hoistFunctionDeclaration(node: FunctionDeclaration): void;
-
-        /**
-         * Hoists a variable declaration to the containing scope.
-         */
-        hoistVariableDeclaration(node: Identifier): void;
-
-        /**
-         * Enables expression substitutions in the pretty printer for the provided SyntaxKind.
-         */
-        enableSubstitution(kind: SyntaxKind): void;
-
-        /**
-         * Determines whether expression substitutions are enabled for the provided node.
-         */
-        isSubstitutionEnabled(node: Node): boolean;
-
-        /**
-         * Hook used by transformers to substitute expressions just before they
-         * are emitted by the pretty printer.
-         */
-        onSubstituteNode?: (emitContext: EmitContext, node: Node) => Node;
-
-        /**
-         * Enables before/after emit notifications in the pretty printer for the provided
-         * SyntaxKind.
-         */
-        enableEmitNotification(kind: SyntaxKind): void;
-
-        /**
-         * Determines whether before/after emit notifications should be raised in the pretty
-         * printer when it emits a node.
-         */
-        isEmitNotificationEnabled(node: Node): boolean;
-
-        /**
-         * Hook used to allow transformers to capture state before or after
-         * the printer emits a node.
-         */
-        onEmitNode?: (emitContext: EmitContext, node: Node, emitCallback: (emitContext: EmitContext, node: Node) => void) => void;
-    }
-
-    /* @internal */
-    export type Transformer = (context: TransformationContext) => (node: SourceFile) => SourceFile;
-
    export function getTransformers(compilerOptions: CompilerOptions) {
        const jsx = compilerOptions.jsx;
        const languageVersion = getEmitScriptTarget(compilerOptions);
@@ -158,13 +80,15 @@ namespace ts {

        let lexicalEnvironmentDisabled = false;

-        let lexicalEnvironmentStackOffset = 0;
        let lexicalEnvironmentVariableDeclarations: VariableDeclaration[];
        let lexicalEnvironmentFunctionDeclarations: FunctionDeclaration[];
        let lexicalEnvironmentVariableDeclarationsStack: VariableDeclaration[][] = [];
        let lexicalEnvironmentFunctionDeclarationsStack: FunctionDeclaration[][] = [];
+        let lexicalEnvironmentStackOffset = 0;
        let lexicalEnvironmentSuspended = false;

+        let emitHelpers: EmitHelper[];
+
        // The transformation context is provided to each transformer as part of transformer
        // initialization.
        const context: TransformationContext = {
@@ -177,6 +101,8 @@ namespace ts {
            endLexicalEnvironment,
            hoistVariableDeclaration,
            hoistFunctionDeclaration,
+            requestEmitHelper,
+            readEmitHelpers,
            onSubstituteNode: (_emitContext, node) => node,
            enableSubstitution,
            isSubstitutionEnabled,
@@ -339,7 +265,7 @@ namespace ts {
        /** Resumes a suspended lexical environment, usually before visiting a function body. */
        function resumeLexicalEnvironment(): void {
            Debug.assert(!lexicalEnvironmentDisabled, "Cannot resume a lexical environment during the print phase.");
-            Debug.assert(lexicalEnvironmentSuspended, "Lexical environment is not suspended suspended.");
+            Debug.assert(lexicalEnvironmentSuspended, "Lexical environment is not suspended.");
            lexicalEnvironmentSuspended = false;
        }

@@ -382,5 +308,18 @@ namespace ts {
            }
            return statements;
        }
+
+        function requestEmitHelper(helper: EmitHelper): void {
+            Debug.assert(!lexicalEnvironmentDisabled, "Cannot modify the lexical environment during the print phase.");
+            Debug.assert(!helper.scoped, "Cannot request a scoped emit helper.");
+            emitHelpers = append(emitHelpers, helper);
+        }
+
+        function readEmitHelpers(): EmitHelper[] | undefined {
+            Debug.assert(!lexicalEnvironmentDisabled, "Cannot modify the lexical environment during the print phase.");
+            const helpers = emitHelpers;
+            emitHelpers = undefined;
+            return helpers;
+        }
    }
 }
@@ -287,7 +287,7 @@ namespace ts {
                    flattenContext.emitBindingOrAssignment(flattenContext.createObjectBindingOrAssignmentPattern(bindingElements), value, location, pattern);
                    bindingElements = undefined;
                }
-                const rhsValue = createRestCall(value, elements, computedTempVariables, pattern);
+                const rhsValue = createRestCall(flattenContext.context, value, elements, computedTempVariables, pattern);
                flattenBindingOrAssignmentElement(flattenContext, element, rhsValue, element);
            }
        }
@@ -451,9 +451,25 @@ namespace ts {
        return name;
    }

+    const restHelper: EmitHelper = {
+        name: "typescript:rest",
+        scoped: false,
+        text: `
+            var __rest = (this && this.__rest) || function (s, e) {
+                var t = {};
+                for (var p in s) if (Object.prototype.hasOwnProperty.call(s, p) && e.indexOf(p) < 0)
+                    t[p] = s[p];
+                if (typeof Object.getOwnPropertySymbols === "function")
+                    for (var i = 0, p = Object.getOwnPropertySymbols(s); i < p.length; i++) if (e.indexOf(p[i]) < 0)
+                        t[p[i]] = s[p[i]];
+                return t;
+            };`
+    };
+
    /** Given value: o, propName: p, pattern: { a, b, ...p } from the original statement
     * `{ a, b, ...p } = o`, create `p = __rest(o, ["a", "b"]);`*/
-    function createRestCall(value: Expression, elements: BindingOrAssignmentElement[], computedTempVariables: Expression[], location: TextRange): Expression {
+    function createRestCall(context: TransformationContext, value: Expression, elements: BindingOrAssignmentElement[], computedTempVariables: Expression[], location: TextRange): Expression {
+        context.requestEmitHelper(restHelper);
        const propertyNames: Expression[] = [];
        let computedTempVariableOffset = 0;
        for (let i = 0; i < elements.length - 1; i++) {
@@ -476,6 +492,6 @@ namespace ts {
                }
            }
        }
-        return createCall(createIdentifier("__rest"), undefined, [value, createArrayLiteral(propertyNames, location)]);
+        return createCall(getHelperName("__rest"), undefined, [value, createArrayLiteral(propertyNames, location)]);
    }
 }
@@ -3,7 +3,6 @@

 /*@internal*/
 namespace ts {
-
    const enum ES2015SubstitutionFlags {
        /** Enables substitutions for captured `this` */
        CapturedThis = 1 << 0,
@@ -210,7 +209,13 @@ namespace ts {

            currentSourceFile = node;
            currentText = node.text;
-            return visitNode(node, visitor, isSourceFile);
+
+            const visited = saveStateAndInvoke(node, visitSourceFile);
+            addEmitHelpers(visited, context.readEmitHelpers());
+
+            currentSourceFile = undefined;
+            currentText = undefined;
+            return visited;
        }

        function visitor(node: Node): VisitResult<Node> {
@@ -256,6 +261,47 @@ namespace ts {
            return visited;
        }

+        function onBeforeVisitNode(node: Node) {
+            if (currentNode) {
+                if (isBlockScope(currentNode, currentParent)) {
+                    enclosingBlockScopeContainer = currentNode;
+                    enclosingBlockScopeContainerParent = currentParent;
+                }
+
+                if (isFunctionLike(currentNode)) {
+                    enclosingFunction = currentNode;
+                    if (currentNode.kind !== SyntaxKind.ArrowFunction) {
+                        enclosingNonArrowFunction = currentNode;
+                        if (!(getEmitFlags(currentNode) & EmitFlags.AsyncFunctionBody)) {
+                            enclosingNonAsyncFunctionBody = currentNode;
+                        }
+                    }
+                }
+
+                // keep track of the enclosing variable statement when in the context of
+                // variable statements, variable declarations, binding elements, and binding
+                // patterns.
+                switch (currentNode.kind) {
+                    case SyntaxKind.VariableStatement:
+                        enclosingVariableStatement = <VariableStatement>currentNode;
+                        break;
+
+                    case SyntaxKind.VariableDeclarationList:
+                    case SyntaxKind.VariableDeclaration:
+                    case SyntaxKind.BindingElement:
+                    case SyntaxKind.ObjectBindingPattern:
+                    case SyntaxKind.ArrayBindingPattern:
+                        break;
+
+                    default:
+                        enclosingVariableStatement = undefined;
+                }
+            }
+
+            currentParent = currentNode;
+            currentNode = node;
+        }
+
        function returnCapturedThis(node: Node): Node {
            return setOriginalNode(createReturn(createIdentifier("_this")), node);
        }
@@ -418,6 +464,9 @@ namespace ts {
                case SyntaxKind.YieldExpression:
                    return visitYieldExpression(<YieldExpression>node);

+                case SyntaxKind.SpreadElement:
+                    return visitSpreadElement(<SpreadElement>node);
+
                case SyntaxKind.SuperKeyword:
                    return visitSuperKeyword();

@@ -428,9 +477,6 @@ namespace ts {
                case SyntaxKind.MethodDeclaration:
                    return visitMethodDeclaration(<MethodDeclaration>node);

-                case SyntaxKind.SourceFile:
-                    return visitSourceFileNode(<SourceFile>node);
-
                case SyntaxKind.VariableStatement:
                    return visitVariableStatement(<VariableStatement>node);

@@ -438,48 +484,19 @@ namespace ts {
                    Debug.failBadSyntaxKind(node);
                    return visitEachChild(node, visitor, context);
            }
-
        }

-        function onBeforeVisitNode(node: Node) {
-            if (currentNode) {
-                if (isBlockScope(currentNode, currentParent)) {
-                    enclosingBlockScopeContainer = currentNode;
-                    enclosingBlockScopeContainerParent = currentParent;
-                }
-
-                if (isFunctionLike(currentNode)) {
-                    enclosingFunction = currentNode;
-                    if (currentNode.kind !== SyntaxKind.ArrowFunction) {
-                        enclosingNonArrowFunction = currentNode;
-                        if (!(getEmitFlags(currentNode) & EmitFlags.AsyncFunctionBody)) {
-                            enclosingNonAsyncFunctionBody = currentNode;
-                        }
-                    }
-                }
-
-                // keep track of the enclosing variable statement when in the context of
-                // variable statements, variable declarations, binding elements, and binding
-                // patterns.
-                switch (currentNode.kind) {
-                    case SyntaxKind.VariableStatement:
-                        enclosingVariableStatement = <VariableStatement>currentNode;
-                        break;
-
-                    case SyntaxKind.VariableDeclarationList:
-                    case SyntaxKind.VariableDeclaration:
-                    case SyntaxKind.BindingElement:
-                    case SyntaxKind.ObjectBindingPattern:
-                    case SyntaxKind.ArrayBindingPattern:
-                        break;
-
-                    default:
-                        enclosingVariableStatement = undefined;
-                }
-            }
-
-            currentParent = currentNode;
-            currentNode = node;
+        function visitSourceFile(node: SourceFile): SourceFile {
+            const statements: Statement[] = [];
+            startLexicalEnvironment();
+            const statementOffset = addPrologueDirectives(statements, node.statements, /*ensureUseStrict*/ false, visitor);
+            addCaptureThisForNodeIfNeeded(statements, node);
+            addRange(statements, visitNodes(node.statements, visitor, isStatement, statementOffset));
+            addRange(statements, endLexicalEnvironment());
+            return updateSourceFileNode(
+                node,
+                createNodeArray(statements, node.statements)
+            );
        }

        function visitSwitchStatement(node: SwitchStatement): SwitchStatement {
@@ -779,7 +796,7 @@ namespace ts {
            if (extendsClauseElement) {
                statements.push(
                    createStatement(
-                        createExtendsHelper(currentSourceFile.externalHelpersModuleName, getLocalName(node)),
+                        createExtendsHelper(context, getLocalName(node)),
                        /*location*/ extendsClauseElement
                    )
                );
@@ -851,7 +868,7 @@ namespace ts {
                // If a super call has already been synthesized,
                // we're going to assume that we should just transform everything after that.
                // The assumption is that no prior step in the pipeline has added any prologue directives.
-                statementOffset = 1;
+                statementOffset = 0;
            }
            else if (constructor) {
                // Otherwise, try to emit all potential prologue directives first.
@@ -1391,20 +1408,13 @@ namespace ts {
        function transformClassMethodDeclarationToStatement(receiver: LeftHandSideExpression, member: MethodDeclaration) {
            const commentRange = getCommentRange(member);
            const sourceMapRange = getSourceMapRange(member);
-
-            const func = transformFunctionLikeToExpression(member, /*location*/ member, /*name*/ undefined);
-            setEmitFlags(func, EmitFlags.NoComments);
-            setSourceMapRange(func, sourceMapRange);
+            const memberName = createMemberAccessForPropertyName(receiver, visitNode(member.name, visitor, isPropertyName), /*location*/ member.name);
+            const memberFunction = transformFunctionLikeToExpression(member, /*location*/ member, /*name*/ undefined);
+            setEmitFlags(memberFunction, EmitFlags.NoComments);
+            setSourceMapRange(memberFunction, sourceMapRange);

            const statement = createStatement(
-                createAssignment(
-                    createMemberAccessForPropertyName(
-                        receiver,
-                        visitNode(member.name, visitor, isPropertyName),
-                        /*location*/ member.name
-                    ),
-                    func
-                ),
+                createAssignment(memberName, memberFunction),
                /*location*/ member
            );

@@ -1682,7 +1692,7 @@ namespace ts {
         *
         * @param node An ExpressionStatement node.
         */
-        function visitExpressionStatement(node: ExpressionStatement): ExpressionStatement {
+        function visitExpressionStatement(node: ExpressionStatement): Statement {
            // If we are here it is most likely because our expression is a destructuring assignment.
            switch (node.expression.kind) {
                case SyntaxKind.ParenthesizedExpression:
@@ -1722,13 +1732,14 @@ namespace ts {
         */
        function visitBinaryExpression(node: BinaryExpression, needsDestructuringValue: boolean): Expression {
            // If we are here it is because this is a destructuring assignment.
-            Debug.assert(isDestructuringAssignment(node));
-            return flattenDestructuringAssignment(
-                <DestructuringAssignment>node,
-                visitor,
-                context,
-                FlattenLevel.All,
-                needsDestructuringValue);
+            if (isDestructuringAssignment(node)) {
+                return flattenDestructuringAssignment(
+                    <DestructuringAssignment>node,
+                    visitor,
+                    context,
+                    FlattenLevel.All,
+                    needsDestructuringValue);
+            }
        }

        function visitVariableStatement(node: VariableStatement): Statement {
@@ -2927,6 +2938,10 @@ namespace ts {
            );
        }

+        function visitSpreadElement(node: SpreadElement) {
+            return visitNode(node.expression, visitor, isExpression);
+        }
+
        /**
         * Transforms the expression of a SpreadExpression node.
         *
@@ -3113,19 +3128,6 @@ namespace ts {
                    : createIdentifier("_super");
        }

-        function visitSourceFileNode(node: SourceFile): SourceFile {
-            const [prologue, remaining] = span(node.statements, isPrologueDirective);
-            const statements: Statement[] = [];
-            startLexicalEnvironment();
-            addRange(statements, prologue);
-            addCaptureThisForNodeIfNeeded(statements, node);
-            addRange(statements, visitNodes(createNodeArray(remaining), visitor, isStatement));
-            addRange(statements, endLexicalEnvironment());
-            const clone = getMutableClone(node);
-            clone.statements = createNodeArray(statements, /*location*/ node.statements);
-            return clone;
-        }
-
        /**
         * Called by the printer just before a node is printed.
         *
@@ -3287,8 +3289,7 @@ namespace ts {
                return false;
            }

-            const parameter = singleOrUndefined(constructor.parameters);
-            if (!parameter || !nodeIsSynthesized(parameter) || !parameter.dotDotDotToken) {
+            if (some(constructor.parameters)) {
                return false;
            }

@@ -3313,7 +3314,31 @@ namespace ts {
            }

            const expression = (<SpreadElement>callArgument).expression;
-            return isIdentifier(expression) && expression === parameter.name;
+            return isIdentifier(expression) && expression.text === "arguments";
        }
    }
+
+    function createExtendsHelper(context: TransformationContext, name: Identifier) {
+        context.requestEmitHelper(extendsHelper);
+        return createCall(
+            getHelperName("__extends"),
+            /*typeArguments*/ undefined,
+            [
+                name,
+                createIdentifier("_super")
+            ]
+        );
+    }
+
+    const extendsHelper: EmitHelper = {
+        name: "typescript:extends",
+        scoped: false,
+        priority: 0,
+        text: `
+            var __extends = (this && this.__extends) || function (d, b) {
+                for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
+                function __() { this.constructor = d; }
+                d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
+            };`
+    };
 }
@@ -5,13 +5,12 @@
 namespace ts {
    type SuperContainer = ClassDeclaration | MethodDeclaration | GetAccessorDeclaration | SetAccessorDeclaration | ConstructorDeclaration;

+    const enum ES2017SubstitutionFlags {
+        /** Enables substitutions for async methods with `super` calls. */
+        AsyncMethodsWithSuper = 1 << 0
+    }
+
    export function transformES2017(context: TransformationContext) {
-
-        const enum ES2017SubstitutionFlags {
-            /** Enables substitutions for async methods with `super` calls. */
-            AsyncMethodsWithSuper = 1 << 0
-        }
-
        const {
            startLexicalEnvironment,
            resumeLexicalEnvironment,
@@ -23,7 +22,8 @@ namespace ts {
        const languageVersion = getEmitScriptTarget(compilerOptions);

        // These variables contain state that changes as we descend into the tree.
-        let currentSourceFileExternalHelpersModuleName: Identifier;
+        let currentSourceFile: SourceFile;
+
        /**
         * Keeps track of whether expression substitution has been enabled for specific edge cases.
         * They are persisted between each SourceFile transformation and should not be reset.
@@ -51,8 +51,13 @@ namespace ts {
                return node;
            }

-            currentSourceFileExternalHelpersModuleName = node.externalHelpersModuleName;
-            return visitEachChild(node, visitor, context);
+            currentSourceFile = node;
+
+            const visited = visitEachChild(node, visitor, context);
+            addEmitHelpers(visited, context.readEmitHelpers());
+
+            currentSourceFile = undefined;
+            return visited;
        }

        function visitor(node: Node): VisitResult<Node> {
@@ -91,11 +96,11 @@ namespace ts {
        }

        /**
-         * Visits an await expression.
+         * Visits an AwaitExpression node.
         *
         * This function will be called any time a ES2017 await expression is encountered.
         *
-         * @param node The await expression node.
+         * @param node The node to visit.
         */
        function visitAwaitExpression(node: AwaitExpression): Expression {
            return setOriginalNode(
@@ -109,12 +114,12 @@ namespace ts {
        }

        /**
-         * Visits a method declaration of a class.
+         * Visits a MethodDeclaration node.
         *
         * This function will be called when one of the following conditions are met:
         * - The node is marked as async
         *
-         * @param node The method node.
+         * @param node The node to visit.
         */
        function visitMethodDeclaration(node: MethodDeclaration) {
            return updateMethod(
@@ -132,12 +137,12 @@ namespace ts {
        }

        /**
-         * Visits a function declaration.
+         * Visits a FunctionDeclaration node.
         *
         * This function will be called when one of the following conditions are met:
         * - The node is marked async
         *
-         * @param node The function node.
+         * @param node The node to visit.
         */
        function visitFunctionDeclaration(node: FunctionDeclaration): VisitResult<Statement> {
            return updateFunctionDeclaration(
@@ -155,12 +160,12 @@ namespace ts {
        }

        /**
-         * Visits a function expression node.
+         * Visits a FunctionExpression node.
         *
         * This function will be called when one of the following conditions are met:
         * - The node is marked async
         *
-         * @param node The function expression node.
+         * @param node The node to visit.
         */
        function visitFunctionExpression(node: FunctionExpression): Expression {
            if (nodeIsMissing(node.body)) {
@@ -180,9 +185,12 @@ namespace ts {
        }

        /**
-         * @remarks
+         * Visits an ArrowFunction.
+         *
         * This function will be called when one of the following conditions are met:
         * - The node is marked async
+         *
+         * @param node The node to visit.
         */
        function visitArrowFunction(node: ArrowFunction) {
            return updateArrowFunction(
@@ -220,7 +228,7 @@ namespace ts {
                statements.push(
                    createReturn(
                        createAwaiterHelper(
-                            currentSourceFileExternalHelpersModuleName,
+                            context,
                            hasLexicalArguments,
                            promiseConstructor,
                            transformFunctionBodyWorker(<Block>node.body, statementOffset)
@@ -229,6 +237,7 @@ namespace ts {
                );

                addRange(statements, endLexicalEnvironment());
+
                const block = createBlock(statements, /*location*/ node.body, /*multiLine*/ true);

                // Minor optimization, emit `_super` helper to capture `super` access in an arrow.
@@ -236,11 +245,11 @@ namespace ts {
                if (languageVersion >= ScriptTarget.ES2015) {
                    if (resolver.getNodeCheckFlags(node) & NodeCheckFlags.AsyncMethodWithSuperBinding) {
                        enableSubstitutionForAsyncMethodsWithSuper();
-                        setEmitFlags(block, EmitFlags.EmitAdvancedSuperHelper);
+                        addEmitHelper(block, advancedAsyncSuperHelper);
                    }
                    else if (resolver.getNodeCheckFlags(node) & NodeCheckFlags.AsyncMethodWithSuper) {
                        enableSubstitutionForAsyncMethodsWithSuper();
-                        setEmitFlags(block, EmitFlags.EmitSuperHelper);
+                        addEmitHelper(block, asyncSuperHelper);
                    }
                }

@@ -248,7 +257,7 @@ namespace ts {
            }
            else {
                const expression = createAwaiterHelper(
-                    currentSourceFileExternalHelpersModuleName,
+                    context,
                    hasLexicalArguments,
                    promiseConstructor,
                    transformFunctionBodyWorker(node.body)
@@ -277,16 +286,15 @@ namespace ts {
        }

        function getPromiseConstructor(type: TypeNode) {
-            if (type) {
-                const typeName = getEntityNameFromTypeNode(type);
-                if (typeName && isEntityName(typeName)) {
-                    const serializationKind = resolver.getTypeReferenceSerializationKind(typeName);
-                    if (serializationKind === TypeReferenceSerializationKind.TypeWithConstructSignatureAndValue
-                        || serializationKind === TypeReferenceSerializationKind.Unknown) {
-                        return typeName;
-                    }
+            const typeName = type && getEntityNameFromTypeNode(type);
+            if (typeName && isEntityName(typeName)) {
+                const serializationKind = resolver.getTypeReferenceSerializationKind(typeName);
+                if (serializationKind === TypeReferenceSerializationKind.TypeWithConstructSignatureAndValue
+                    || serializationKind === TypeReferenceSerializationKind.Unknown) {
+                    return typeName;
                }
            }
+
            return undefined;
        }

@@ -448,4 +456,63 @@ namespace ts {
                && resolver.getNodeCheckFlags(currentSuperContainer) & (NodeCheckFlags.AsyncMethodWithSuper | NodeCheckFlags.AsyncMethodWithSuperBinding);
        }
    }
+
+    function createAwaiterHelper(context: TransformationContext, hasLexicalArguments: boolean, promiseConstructor: EntityName | Expression, body: Block) {
+        context.requestEmitHelper(awaiterHelper);
+        const generatorFunc = createFunctionExpression(
+            /*modifiers*/ undefined,
+            createToken(SyntaxKind.AsteriskToken),
+            /*name*/ undefined,
+            /*typeParameters*/ undefined,
+            /*parameters*/ [],
+            /*type*/ undefined,
+            body
+        );
+
+        // Mark this node as originally an async function
+        (generatorFunc.emitNode || (generatorFunc.emitNode = {})).flags |= EmitFlags.AsyncFunctionBody;
+
+        return createCall(
+            getHelperName("__awaiter"),
+            /*typeArguments*/ undefined,
+            [
+                createThis(),
+                hasLexicalArguments ? createIdentifier("arguments") : createVoidZero(),
+                promiseConstructor ? createExpressionFromEntityName(promiseConstructor) : createVoidZero(),
+                generatorFunc
+            ]
+        );
+    }
+
+    const awaiterHelper: EmitHelper = {
+        name: "typescript:awaiter",
+        scoped: false,
+        priority: 5,
+        text: `
+            var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {
+                return new (P || (P = Promise))(function (resolve, reject) {
+                    function fulfilled(value) { try { step(generator.next(value)); } catch (e) { reject(e); } }
+                    function rejected(value) { try { step(generator["throw"](value)); } catch (e) { reject(e); } }
+                    function step(result) { result.done ? resolve(result.value) : new P(function (resolve) { resolve(result.value); }).then(fulfilled, rejected); }
+                    step((generator = generator.apply(thisArg, _arguments)).next());
+                });
+            };`
+    };
+
+    const asyncSuperHelper: EmitHelper = {
+        name: "typescript:async-super",
+        scoped: true,
+        text: `
+            const _super = name => super[name];`
+    };
+
+    const advancedAsyncSuperHelper: EmitHelper = {
+        name: "typescript:advanced-async-super",
+        scoped: true,
+        text: `
+            const _super = (function (geti, seti) {
+                const cache = Object.create(null);
+                return name => cache[name] || (cache[name] = { get value() { return geti(name); }, set value(v) { seti(name, v); } });
+            })(name => super[name], (name, value) => super[name] = value);`
+    };
 }
@@ -15,7 +15,9 @@ namespace ts {
                return node;
            }

-            return visitEachChild(node, visitor, context);
+            const visited = visitEachChild(node, visitor, context);
+            addEmitHelpers(visited, context.readEmitHelpers());
+            return visited;
        }

        function visitor(node: Node): VisitResult<Node> {
@@ -112,7 +114,7 @@ namespace ts {
                if (objects.length && objects[0].kind !== SyntaxKind.ObjectLiteralExpression) {
                    objects.unshift(createObjectLiteral());
                }
-                return createCall(createIdentifier("__assign"), undefined, objects);
+                return createAssignHelper(context, objects);
            }
            return visitEachChild(node, visitor, context);
        }
@@ -383,4 +385,31 @@ namespace ts {
            return body;
        }
    }
+
+    const assignHelper: EmitHelper = {
+        name: "typescript:assign",
+        scoped: false,
+        priority: 1,
+        text: `
+            var __assign = (this && this.__assign) || Object.assign || function(t) {
+                for (var s, i = 1, n = arguments.length; i < n; i++) {
+                    s = arguments[i];
+                    for (var p in s) if (Object.prototype.hasOwnProperty.call(s, p))
+                        t[p] = s[p];
+                    if (typeof Object.getOwnPropertySymbols === "function")
+                        for (var i = 0, p = Object.getOwnPropertySymbols(s); i < p.length; i++)
+                            t[p[i]] = s[p[i]];
+                }
+                return t;
+            };`
+    };
+
+    export function createAssignHelper(context: TransformationContext, attributesSegments: Expression[]) {
+        context.requestEmitHelper(assignHelper);
+        return createCall(
+            getHelperName("__assign"),
+            /*typeArguments*/ undefined,
+            attributesSegments
+        );
+    }
 }
@@ -291,17 +291,18 @@ namespace ts {
        return transformSourceFile;

        function transformSourceFile(node: SourceFile) {
-            if (isDeclarationFile(node)) {
+            if (isDeclarationFile(node)
+                || (node.transformFlags & TransformFlags.ContainsGenerator) === 0) {
                return node;
            }

-            if (node.transformFlags & TransformFlags.ContainsGenerator) {
-                currentSourceFile = node;
-                node = visitEachChild(node, visitor, context);
-                currentSourceFile = undefined;
-            }
+            currentSourceFile = node;

-            return node;
+            const visited = visitEachChild(node, visitor, context);
+            addEmitHelpers(visited, context.readEmitHelpers());
+
+            currentSourceFile = undefined;
+            return visited;
        }

        /**
@@ -2597,28 +2598,24 @@ namespace ts {
            withBlockStack = undefined;

            const buildResult = buildStatements();
-            return createCall(
-                createHelperName(currentSourceFile.externalHelpersModuleName, "__generator"),
-                /*typeArguments*/ undefined,
-                [
-                    createThis(),
-                    setEmitFlags(
-                        createFunctionExpression(
-                            /*modifiers*/ undefined,
-                            /*asteriskToken*/ undefined,
-                            /*name*/ undefined,
-                            /*typeParameters*/ undefined,
-                            [createParameter(/*decorators*/ undefined, /*modifiers*/ undefined, /*dotDotDotToken*/ undefined, state)],
-                            /*type*/ undefined,
-                            createBlock(
-                                buildResult,
-                                /*location*/ undefined,
-                                /*multiLine*/ buildResult.length > 0
-                            )
-                        ),
-                        EmitFlags.ReuseTempVariableScope
-                    )
-                ]
+            return createGeneratorHelper(
+                context,
+                setEmitFlags(
+                    createFunctionExpression(
+                        /*modifiers*/ undefined,
+                        /*asteriskToken*/ undefined,
+                        /*name*/ undefined,
+                        /*typeParameters*/ undefined,
+                        [createParameter(/*decorators*/ undefined, /*modifiers*/ undefined, /*dotDotDotToken*/ undefined, state)],
+                        /*type*/ undefined,
+                        createBlock(
+                            buildResult,
+                            /*location*/ undefined,
+                            /*multiLine*/ buildResult.length > 0
+                        )
+                    ),
+                    EmitFlags.ReuseTempVariableScope
+                )
            );
        }

@@ -3094,4 +3091,105 @@ namespace ts {
            );
        }
    }
+
+    function createGeneratorHelper(context: TransformationContext, body: FunctionExpression) {
+        context.requestEmitHelper(generatorHelper);
+        return createCall(
+            getHelperName("__generator"),
+            /*typeArguments*/ undefined,
+            [createThis(), body]);
+    }
+
+    // The __generator helper is used by down-level transformations to emulate the runtime
+    // semantics of an ES2015 generator function. When called, this helper returns an
+    // object that implements the Iterator protocol, in that it has `next`, `return`, and
+    // `throw` methods that step through the generator when invoked.
+    //
+    // parameters:
+    //  thisArg  The value to use as the `this` binding for the transformed generator body.
+    //  body     A function that acts as the transformed generator body.
+    //
+    // variables:
+    //  _       Persistent state for the generator that is shared between the helper and the
+    //          generator body. The state object has the following members:
+    //            sent() - A method that returns or throws the current completion value.
+    //            label  - The next point at which to resume evaluation of the generator body.
+    //            trys   - A stack of protected regions (try/catch/finally blocks).
+    //            ops    - A stack of pending instructions when inside of a finally block.
+    //  f       A value indicating whether the generator is executing.
+    //  y       An iterator to delegate for a yield*.
+    //  t       A temporary variable that holds one of the following values (note that these
+    //          cases do not overlap):
+    //          - The completion value when resuming from a `yield` or `yield*`.
+    //          - The error value for a catch block.
+    //          - The current protected region (array of try/catch/finally/end labels).
+    //          - The verb (`next`, `throw`, or `return` method) to delegate to the expression
+    //            of a `yield*`.
+    //          - The result of evaluating the verb delegated to the expression of a `yield*`.
+    //
+    // functions:
+    //  verb(n)     Creates a bound callback to the `step` function for opcode `n`.
+    //  step(op)    Evaluates opcodes in a generator body until execution is suspended or
+    //              completed.
+    //
+    // The __generator helper understands a limited set of instructions:
+    //  0: next(value?)     - Start or resume the generator with the specified value.
+    //  1: throw(error)     - Resume the generator with an exception. If the generator is
+    //                        suspended inside of one or more protected regions, evaluates
+    //                        any intervening finally blocks between the current label and
+    //                        the nearest catch block or function boundary. If uncaught, the
+    //                        exception is thrown to the caller.
+    //  2: return(value?)   - Resume the generator as if with a return. If the generator is
+    //                        suspended inside of one or more protected regions, evaluates any
+    //                        intervening finally blocks.
+    //  3: break(label)     - Jump to the specified label. If the label is outside of the
+    //                        current protected region, evaluates any intervening finally
+    //                        blocks.
+    //  4: yield(value?)    - Yield execution to the caller with an optional value. When
+    //                        resumed, the generator will continue at the next label.
+    //  5: yield*(value)    - Delegates evaluation to the supplied iterator. When
+    //                        delegation completes, the generator will continue at the next
+    //                        label.
+    //  6: catch(error)     - Handles an exception thrown from within the generator body. If
+    //                        the current label is inside of one or more protected regions,
+    //                        evaluates any intervening finally blocks between the current
+    //                        label and the nearest catch block or function boundary. If
+    //                        uncaught, the exception is thrown to the caller.
+    //  7: endfinally       - Ends a finally block, resuming the last instruction prior to
+    //                        entering a finally block.
+    //
+    // For examples of how these are used, see the comments in ./transformers/generators.ts
+    const generatorHelper: EmitHelper = {
+        name: "typescript:generator",
+        scoped: false,
+        priority: 6,
+        text: `
+            var __generator = (this && this.__generator) || function (thisArg, body) {
+                var _ = { label: 0, sent: function() { if (t[0] & 1) throw t[1]; return t[1]; }, trys: [], ops: [] }, f, y, t;
+                return { next: verb(0), "throw": verb(1), "return": verb(2) };
+                function verb(n) { return function (v) { return step([n, v]); }; }
+                function step(op) {
+                    if (f) throw new TypeError("Generator is already executing.");
+                    while (_) try {
+                        if (f = 1, y && (t = y[op[0] & 2 ? "return" : op[0] ? "throw" : "next"]) && !(t = t.call(y, op[1])).done) return t;
+                        if (y = 0, t) op = [0, t.value];
+                        switch (op[0]) {
+                            case 0: case 1: t = op; break;
+                            case 4: _.label++; return { value: op[1], done: false };
+                            case 5: _.label++; y = op[1]; op = [0]; continue;
+                            case 7: op = _.ops.pop(); _.trys.pop(); continue;
+                            default:
+                                if (!(t = _.trys, t = t.length > 0 && t[t.length - 1]) && (op[0] === 6 || op[0] === 2)) { _ = 0; continue; }
+                                if (op[0] === 3 && (!t || (op[1] > t[0] && op[1] < t[3]))) { _.label = op[1]; break; }
+                                if (op[0] === 6 && _.label < t[1]) { _.label = t[1]; t = op; break; }
+                                if (t && _.label < t[2]) { _.label = t[2]; _.ops.push(op); break; }
+                                if (t[2]) _.ops.pop();
+                                _.trys.pop(); continue;
+                        }
+                        op = body.call(thisArg, _);
+                    } catch (e) { op = [6, e]; y = 0; } finally { f = t = 0; }
+                    if (op[0] & 5) throw op[1]; return { value: op[0] ? op[1] : void 0, done: true };
+                }
+            };`
+    };
 }
@@ -1,13 +1,13 @@
 /// <reference path="../factory.ts" />
 /// <reference path="../visitor.ts" />
+/// <reference path="./esnext.ts" />

 /*@internal*/
 namespace ts {
-    const entities: Map<number> = createEntitiesMap();
-
    export function transformJsx(context: TransformationContext) {
        const compilerOptions = context.getCompilerOptions();
        let currentSourceFile: SourceFile;
+
        return transformSourceFile;

        /**
@@ -21,9 +21,12 @@ namespace ts {
            }

            currentSourceFile = node;
-            node = visitEachChild(node, visitor, context);
+
+            const visited = visitEachChild(node, visitor, context);
+            addEmitHelpers(visited, context.readEmitHelpers());
+
            currentSourceFile = undefined;
-            return node;
+            return visited;
        }

        function visitor(node: Node): VisitResult<Node> {
@@ -105,8 +108,10 @@ namespace ts {

                // Either emit one big object literal (no spread attribs), or
                // a call to the __assign helper.
-                objectProperties = singleOrUndefined(segments)
-                    || createAssignHelper(currentSourceFile.externalHelpersModuleName, segments);
+                objectProperties = singleOrUndefined(segments);
+                if (!objectProperties) {
+                    objectProperties = createAssignHelper(context, segments);
+                }
            }

            const element = createExpressionForJsxElement(
@@ -272,261 +277,259 @@ namespace ts {
        }
    }

-    function createEntitiesMap(): Map<number> {
-        return createMap<number>({
-            "quot": 0x0022,
-            "amp": 0x0026,
-            "apos": 0x0027,
-            "lt": 0x003C,
-            "gt": 0x003E,
-            "nbsp": 0x00A0,
-            "iexcl": 0x00A1,
-            "cent": 0x00A2,
-            "pound": 0x00A3,
-            "curren": 0x00A4,
-            "yen": 0x00A5,
-            "brvbar": 0x00A6,
-            "sect": 0x00A7,
-            "uml": 0x00A8,
-            "copy": 0x00A9,
-            "ordf": 0x00AA,
-            "laquo": 0x00AB,
-            "not": 0x00AC,
-            "shy": 0x00AD,
-            "reg": 0x00AE,
-            "macr": 0x00AF,
-            "deg": 0x00B0,
-            "plusmn": 0x00B1,
-            "sup2": 0x00B2,
-            "sup3": 0x00B3,
-            "acute": 0x00B4,
-            "micro": 0x00B5,
-            "para": 0x00B6,
-            "middot": 0x00B7,
-            "cedil": 0x00B8,
-            "sup1": 0x00B9,
-            "ordm": 0x00BA,
-            "raquo": 0x00BB,
-            "frac14": 0x00BC,
-            "frac12": 0x00BD,
-            "frac34": 0x00BE,
-            "iquest": 0x00BF,
-            "Agrave": 0x00C0,
-            "Aacute": 0x00C1,
-            "Acirc": 0x00C2,
-            "Atilde": 0x00C3,
-            "Auml": 0x00C4,
-            "Aring": 0x00C5,
-            "AElig": 0x00C6,
-            "Ccedil": 0x00C7,
-            "Egrave": 0x00C8,
-            "Eacute": 0x00C9,
-            "Ecirc": 0x00CA,
-            "Euml": 0x00CB,
-            "Igrave": 0x00CC,
-            "Iacute": 0x00CD,
-            "Icirc": 0x00CE,
-            "Iuml": 0x00CF,
-            "ETH": 0x00D0,
-            "Ntilde": 0x00D1,
-            "Ograve": 0x00D2,
-            "Oacute": 0x00D3,
-            "Ocirc": 0x00D4,
-            "Otilde": 0x00D5,
-            "Ouml": 0x00D6,
-            "times": 0x00D7,
-            "Oslash": 0x00D8,
-            "Ugrave": 0x00D9,
-            "Uacute": 0x00DA,
-            "Ucirc": 0x00DB,
-            "Uuml": 0x00DC,
-            "Yacute": 0x00DD,
-            "THORN": 0x00DE,
-            "szlig": 0x00DF,
-            "agrave": 0x00E0,
-            "aacute": 0x00E1,
-            "acirc": 0x00E2,
-            "atilde": 0x00E3,
-            "auml": 0x00E4,
-            "aring": 0x00E5,
-            "aelig": 0x00E6,
-            "ccedil": 0x00E7,
-            "egrave": 0x00E8,
-            "eacute": 0x00E9,
-            "ecirc": 0x00EA,
-            "euml": 0x00EB,
-            "igrave": 0x00EC,
-            "iacute": 0x00ED,
-            "icirc": 0x00EE,
-            "iuml": 0x00EF,
-            "eth": 0x00F0,
-            "ntilde": 0x00F1,
-            "ograve": 0x00F2,
-            "oacute": 0x00F3,
-            "ocirc": 0x00F4,
-            "otilde": 0x00F5,
-            "ouml": 0x00F6,
-            "divide": 0x00F7,
-            "oslash": 0x00F8,
-            "ugrave": 0x00F9,
-            "uacute": 0x00FA,
-            "ucirc": 0x00FB,
-            "uuml": 0x00FC,
-            "yacute": 0x00FD,
-            "thorn": 0x00FE,
-            "yuml": 0x00FF,
-            "OElig": 0x0152,
-            "oelig": 0x0153,
-            "Scaron": 0x0160,
-            "scaron": 0x0161,
-            "Yuml": 0x0178,
-            "fnof": 0x0192,
-            "circ": 0x02C6,
-            "tilde": 0x02DC,
-            "Alpha": 0x0391,
-            "Beta": 0x0392,
-            "Gamma": 0x0393,
-            "Delta": 0x0394,
-            "Epsilon": 0x0395,
-            "Zeta": 0x0396,
-            "Eta": 0x0397,
-            "Theta": 0x0398,
-            "Iota": 0x0399,
-            "Kappa": 0x039A,
-            "Lambda": 0x039B,
-            "Mu": 0x039C,
-            "Nu": 0x039D,
-            "Xi": 0x039E,
-            "Omicron": 0x039F,
-            "Pi": 0x03A0,
-            "Rho": 0x03A1,
-            "Sigma": 0x03A3,
-            "Tau": 0x03A4,
-            "Upsilon": 0x03A5,
-            "Phi": 0x03A6,
-            "Chi": 0x03A7,
-            "Psi": 0x03A8,
-            "Omega": 0x03A9,
-            "alpha": 0x03B1,
-            "beta": 0x03B2,
-            "gamma": 0x03B3,
-            "delta": 0x03B4,
-            "epsilon": 0x03B5,
-            "zeta": 0x03B6,
-            "eta": 0x03B7,
-            "theta": 0x03B8,
-            "iota": 0x03B9,
-            "kappa": 0x03BA,
-            "lambda": 0x03BB,
-            "mu": 0x03BC,
-            "nu": 0x03BD,
-            "xi": 0x03BE,
-            "omicron": 0x03BF,
-            "pi": 0x03C0,
-            "rho": 0x03C1,
-            "sigmaf": 0x03C2,
-            "sigma": 0x03C3,
-            "tau": 0x03C4,
-            "upsilon": 0x03C5,
-            "phi": 0x03C6,
-            "chi": 0x03C7,
-            "psi": 0x03C8,
-            "omega": 0x03C9,
-            "thetasym": 0x03D1,
-            "upsih": 0x03D2,
-            "piv": 0x03D6,
-            "ensp": 0x2002,
-            "emsp": 0x2003,
-            "thinsp": 0x2009,
-            "zwnj": 0x200C,
-            "zwj": 0x200D,
-            "lrm": 0x200E,
-            "rlm": 0x200F,
-            "ndash": 0x2013,
-            "mdash": 0x2014,
-            "lsquo": 0x2018,
-            "rsquo": 0x2019,
-            "sbquo": 0x201A,
-            "ldquo": 0x201C,
-            "rdquo": 0x201D,
-            "bdquo": 0x201E,
-            "dagger": 0x2020,
-            "Dagger": 0x2021,
-            "bull": 0x2022,
-            "hellip": 0x2026,
-            "permil": 0x2030,
-            "prime": 0x2032,
-            "Prime": 0x2033,
-            "lsaquo": 0x2039,
-            "rsaquo": 0x203A,
-            "oline": 0x203E,
-            "frasl": 0x2044,
-            "euro": 0x20AC,
-            "image": 0x2111,
-            "weierp": 0x2118,
-            "real": 0x211C,
-            "trade": 0x2122,
-            "alefsym": 0x2135,
-            "larr": 0x2190,
-            "uarr": 0x2191,
-            "rarr": 0x2192,
-            "darr": 0x2193,
-            "harr": 0x2194,
-            "crarr": 0x21B5,
-            "lArr": 0x21D0,
-            "uArr": 0x21D1,
-            "rArr": 0x21D2,
-            "dArr": 0x21D3,
-            "hArr": 0x21D4,
-            "forall": 0x2200,
-            "part": 0x2202,
-            "exist": 0x2203,
-            "empty": 0x2205,
-            "nabla": 0x2207,
-            "isin": 0x2208,
-            "notin": 0x2209,
-            "ni": 0x220B,
-            "prod": 0x220F,
-            "sum": 0x2211,
-            "minus": 0x2212,
-            "lowast": 0x2217,
-            "radic": 0x221A,
-            "prop": 0x221D,
-            "infin": 0x221E,
-            "ang": 0x2220,
-            "and": 0x2227,
-            "or": 0x2228,
-            "cap": 0x2229,
-            "cup": 0x222A,
-            "int": 0x222B,
-            "there4": 0x2234,
-            "sim": 0x223C,
-            "cong": 0x2245,
-            "asymp": 0x2248,
-            "ne": 0x2260,
-            "equiv": 0x2261,
-            "le": 0x2264,
-            "ge": 0x2265,
-            "sub": 0x2282,
-            "sup": 0x2283,
-            "nsub": 0x2284,
-            "sube": 0x2286,
-            "supe": 0x2287,
-            "oplus": 0x2295,
-            "otimes": 0x2297,
-            "perp": 0x22A5,
-            "sdot": 0x22C5,
-            "lceil": 0x2308,
-            "rceil": 0x2309,
-            "lfloor": 0x230A,
-            "rfloor": 0x230B,
-            "lang": 0x2329,
-            "rang": 0x232A,
-            "loz": 0x25CA,
-            "spades": 0x2660,
-            "clubs": 0x2663,
-            "hearts": 0x2665,
-            "diams": 0x2666
-        });
-    }
+    const entities = createMap<number>({
+        "quot": 0x0022,
+        "amp": 0x0026,
+        "apos": 0x0027,
+        "lt": 0x003C,
+        "gt": 0x003E,
+        "nbsp": 0x00A0,
+        "iexcl": 0x00A1,
+        "cent": 0x00A2,
+        "pound": 0x00A3,
+        "curren": 0x00A4,
+        "yen": 0x00A5,
+        "brvbar": 0x00A6,
+        "sect": 0x00A7,
+        "uml": 0x00A8,
+        "copy": 0x00A9,
+        "ordf": 0x00AA,
+        "laquo": 0x00AB,
+        "not": 0x00AC,
+        "shy": 0x00AD,
+        "reg": 0x00AE,
+        "macr": 0x00AF,
+        "deg": 0x00B0,
+        "plusmn": 0x00B1,
+        "sup2": 0x00B2,
+        "sup3": 0x00B3,
+        "acute": 0x00B4,
+        "micro": 0x00B5,
+        "para": 0x00B6,
+        "middot": 0x00B7,
+        "cedil": 0x00B8,
+        "sup1": 0x00B9,
+        "ordm": 0x00BA,
+        "raquo": 0x00BB,
+        "frac14": 0x00BC,
+        "frac12": 0x00BD,
+        "frac34": 0x00BE,
+        "iquest": 0x00BF,
+        "Agrave": 0x00C0,
+        "Aacute": 0x00C1,
+        "Acirc": 0x00C2,
+        "Atilde": 0x00C3,
+        "Auml": 0x00C4,
+        "Aring": 0x00C5,
+        "AElig": 0x00C6,
+        "Ccedil": 0x00C7,
+        "Egrave": 0x00C8,
+        "Eacute": 0x00C9,
+        "Ecirc": 0x00CA,
+        "Euml": 0x00CB,
+        "Igrave": 0x00CC,
+        "Iacute": 0x00CD,
+        "Icirc": 0x00CE,
+        "Iuml": 0x00CF,
+        "ETH": 0x00D0,
+        "Ntilde": 0x00D1,
+        "Ograve": 0x00D2,
+        "Oacute": 0x00D3,
+        "Ocirc": 0x00D4,
+        "Otilde": 0x00D5,
+        "Ouml": 0x00D6,
+        "times": 0x00D7,
+        "Oslash": 0x00D8,
+        "Ugrave": 0x00D9,
+        "Uacute": 0x00DA,
+        "Ucirc": 0x00DB,
+        "Uuml": 0x00DC,
+        "Yacute": 0x00DD,
+        "THORN": 0x00DE,
+        "szlig": 0x00DF,
+        "agrave": 0x00E0,
+        "aacute": 0x00E1,
+        "acirc": 0x00E2,
+        "atilde": 0x00E3,
+        "auml": 0x00E4,
+        "aring": 0x00E5,
+        "aelig": 0x00E6,
+        "ccedil": 0x00E7,
+        "egrave": 0x00E8,
+        "eacute": 0x00E9,
+        "ecirc": 0x00EA,
+        "euml": 0x00EB,
+        "igrave": 0x00EC,
+        "iacute": 0x00ED,
+        "icirc": 0x00EE,
+        "iuml": 0x00EF,
+        "eth": 0x00F0,
+        "ntilde": 0x00F1,
+        "ograve": 0x00F2,
+        "oacute": 0x00F3,
+        "ocirc": 0x00F4,
+        "otilde": 0x00F5,
+        "ouml": 0x00F6,
+        "divide": 0x00F7,
+        "oslash": 0x00F8,
+        "ugrave": 0x00F9,
+        "uacute": 0x00FA,
+        "ucirc": 0x00FB,
+        "uuml": 0x00FC,
+        "yacute": 0x00FD,
+        "thorn": 0x00FE,
+        "yuml": 0x00FF,
+        "OElig": 0x0152,
+        "oelig": 0x0153,
+        "Scaron": 0x0160,
+        "scaron": 0x0161,
+        "Yuml": 0x0178,
+        "fnof": 0x0192,
+        "circ": 0x02C6,
+        "tilde": 0x02DC,
+        "Alpha": 0x0391,
+        "Beta": 0x0392,
+        "Gamma": 0x0393,
+        "Delta": 0x0394,
+        "Epsilon": 0x0395,
+        "Zeta": 0x0396,
+        "Eta": 0x0397,
+        "Theta": 0x0398,
+        "Iota": 0x0399,
+        "Kappa": 0x039A,
+        "Lambda": 0x039B,
+        "Mu": 0x039C,
+        "Nu": 0x039D,
+        "Xi": 0x039E,
+        "Omicron": 0x039F,
+        "Pi": 0x03A0,
+        "Rho": 0x03A1,
+        "Sigma": 0x03A3,
+        "Tau": 0x03A4,
+        "Upsilon": 0x03A5,
+        "Phi": 0x03A6,
+        "Chi": 0x03A7,
+        "Psi": 0x03A8,
+        "Omega": 0x03A9,
+        "alpha": 0x03B1,
+        "beta": 0x03B2,
+        "gamma": 0x03B3,
+        "delta": 0x03B4,
+        "epsilon": 0x03B5,
+        "zeta": 0x03B6,
+        "eta": 0x03B7,
+        "theta": 0x03B8,
+        "iota": 0x03B9,
+        "kappa": 0x03BA,
+        "lambda": 0x03BB,
+        "mu": 0x03BC,
+        "nu": 0x03BD,
+        "xi": 0x03BE,
+        "omicron": 0x03BF,
+        "pi": 0x03C0,
+        "rho": 0x03C1,
+        "sigmaf": 0x03C2,
+        "sigma": 0x03C3,
+        "tau": 0x03C4,
+        "upsilon": 0x03C5,
+        "phi": 0x03C6,
+        "chi": 0x03C7,
+        "psi": 0x03C8,
+        "omega": 0x03C9,
+        "thetasym": 0x03D1,
+        "upsih": 0x03D2,
+        "piv": 0x03D6,
+        "ensp": 0x2002,
+        "emsp": 0x2003,
+        "thinsp": 0x2009,
+        "zwnj": 0x200C,
+        "zwj": 0x200D,
+        "lrm": 0x200E,
+        "rlm": 0x200F,
+        "ndash": 0x2013,
+        "mdash": 0x2014,
+        "lsquo": 0x2018,
+        "rsquo": 0x2019,
+        "sbquo": 0x201A,
+        "ldquo": 0x201C,
+        "rdquo": 0x201D,
+        "bdquo": 0x201E,
+        "dagger": 0x2020,
+        "Dagger": 0x2021,
+        "bull": 0x2022,
+        "hellip": 0x2026,
+        "permil": 0x2030,
+        "prime": 0x2032,
+        "Prime": 0x2033,
+        "lsaquo": 0x2039,
+        "rsaquo": 0x203A,
+        "oline": 0x203E,
+        "frasl": 0x2044,
+        "euro": 0x20AC,
+        "image": 0x2111,
+        "weierp": 0x2118,
+        "real": 0x211C,
+        "trade": 0x2122,
+        "alefsym": 0x2135,
+        "larr": 0x2190,
+        "uarr": 0x2191,
+        "rarr": 0x2192,
+        "darr": 0x2193,
+        "harr": 0x2194,
+        "crarr": 0x21B5,
+        "lArr": 0x21D0,
+        "uArr": 0x21D1,
+        "rArr": 0x21D2,
+        "dArr": 0x21D3,
+        "hArr": 0x21D4,
+        "forall": 0x2200,
+        "part": 0x2202,
+        "exist": 0x2203,
+        "empty": 0x2205,
+        "nabla": 0x2207,
+        "isin": 0x2208,
+        "notin": 0x2209,
+        "ni": 0x220B,
+        "prod": 0x220F,
+        "sum": 0x2211,
+        "minus": 0x2212,
+        "lowast": 0x2217,
+        "radic": 0x221A,
+        "prop": 0x221D,
+        "infin": 0x221E,
+        "ang": 0x2220,
+        "and": 0x2227,
+        "or": 0x2228,
+        "cap": 0x2229,
+        "cup": 0x222A,
+        "int": 0x222B,
+        "there4": 0x2234,
+        "sim": 0x223C,
+        "cong": 0x2245,
+        "asymp": 0x2248,
+        "ne": 0x2260,
+        "equiv": 0x2261,
+        "le": 0x2264,
+        "ge": 0x2265,
+        "sub": 0x2282,
+        "sup": 0x2283,
+        "nsub": 0x2284,
+        "sube": 0x2286,
+        "supe": 0x2287,
+        "oplus": 0x2295,
+        "otimes": 0x2297,
+        "perp": 0x22A5,
+        "sdot": 0x22C5,
+        "lceil": 0x2308,
+        "rceil": 0x2309,
+        "lfloor": 0x230A,
+        "rfloor": 0x230B,
+        "lang": 0x2329,
+        "rang": 0x232A,
+        "loz": 0x25CA,
+        "spades": 0x2660,
+        "clubs": 0x2663,
+        "hearts": 0x2665,
+        "diams": 0x2666
+    });
 }
@@ -5,6 +5,14 @@
 namespace ts {
    export function transformES2015Module(context: TransformationContext) {
        const compilerOptions = context.getCompilerOptions();
+        const previousOnEmitNode = context.onEmitNode;
+        const previousOnSubstituteNode = context.onSubstituteNode;
+        context.onEmitNode = onEmitNode;
+        context.onSubstituteNode = onSubstituteNode;
+        context.enableEmitNotification(SyntaxKind.SourceFile);
+        context.enableSubstitution(SyntaxKind.Identifier);
+
+        let currentSourceFile: SourceFile;
        return transformSourceFile;

        function transformSourceFile(node: SourceFile) {
@@ -13,7 +21,27 @@ namespace ts {
            }

            if (isExternalModule(node) || compilerOptions.isolatedModules) {
-                return visitEachChild(node, visitor, context);
+                const externalHelpersModuleName = getOrCreateExternalHelpersModuleNameIfNeeded(node, compilerOptions);
+                if (externalHelpersModuleName) {
+                    const statements: Statement[] = [];
+                    const statementOffset = addPrologueDirectives(statements, node.statements);
+                    append(statements,
+                        createImportDeclaration(
+                            /*decorators*/ undefined,
+                            /*modifiers*/ undefined,
+                            createImportClause(/*name*/ undefined, createNamespaceImport(externalHelpersModuleName)),
+                            createLiteral(externalHelpersModuleNameText)
+                        )
+                    );
+
+                    addRange(statements, visitNodes(node.statements, visitor, isStatement, statementOffset));
+                    return updateSourceFileNode(
+                        node,
+                        createNodeArray(statements, node.statements));
+                }
+                else {
+                    return visitEachChild(node, visitor, context);
+                }
            }

            return node;
@@ -35,5 +63,55 @@ namespace ts {
            // Elide `export=` as it is not legal with --module ES6
            return node.isExportEquals ? undefined : node;
        }
+
+        //
+        // Emit Notification
+        //
+
+        /**
+         * Hook for node emit.
+         *
+         * @param emitContext A context hint for the emitter.
+         * @param node The node to emit.
+         * @param emit A callback used to emit the node in the printer.
+         */
+        function onEmitNode(emitContext: EmitContext, node: Node, emitCallback: (emitContext: EmitContext, node: Node) => void): void {
+            if (isSourceFile(node)) {
+                currentSourceFile = node;
+                previousOnEmitNode(emitContext, node, emitCallback);
+                currentSourceFile = undefined;
+            }
+            else {
+                previousOnEmitNode(emitContext, node, emitCallback);
+            }
+        }
+
+        //
+        // Substitutions
+        //
+
+        /**
+         * Hooks node substitutions.
+         *
+         * @param emitContext A context hint for the emitter.
+         * @param node The node to substitute.
+         */
+        function onSubstituteNode(emitContext: EmitContext, node: Node) {
+            node = previousOnSubstituteNode(emitContext, node);
+            if (isIdentifier(node) && emitContext === EmitContext.Expression) {
+                return substituteExpressionIdentifier(node);
+            }
+            return node;
+        }
+
+        function substituteExpressionIdentifier(node: Identifier): Expression {
+            if (getEmitFlags(node) & EmitFlags.HelperName) {
+                const externalHelpersModuleName = getExternalHelpersModuleName(currentSourceFile);
+                if (externalHelpersModuleName) {
+                    return createPropertyAccess(externalHelpersModuleName, node);
+                }
+            }
+            return node;
+        }
    }
 }
@@ -60,7 +60,7 @@ namespace ts {
            }

            currentSourceFile = node;
-            currentModuleInfo = moduleInfoMap[getOriginalNodeId(node)] = collectExternalModuleInfo(node, resolver);
+            currentModuleInfo = moduleInfoMap[getOriginalNodeId(node)] = collectExternalModuleInfo(node, resolver, compilerOptions);

            // Perform the transformation.
            const transformModule = transformModuleDelegates[moduleKind] || transformModuleDelegates[ModuleKind.None];
@@ -81,13 +81,14 @@ namespace ts {

            const statements: Statement[] = [];
            const statementOffset = addPrologueDirectives(statements, node.statements, /*ensureUseStrict*/ !compilerOptions.noImplicitUseStrict, sourceElementVisitor);
+            append(statements, visitNode(currentModuleInfo.externalHelpersImportDeclaration, sourceElementVisitor, isStatement, /*optional*/ true));
            addRange(statements, visitNodes(node.statements, sourceElementVisitor, isStatement, statementOffset));
            addRange(statements, endLexicalEnvironment());
            addExportEqualsIfNeeded(statements, /*emitAsReturn*/ false);

            const updated = updateSourceFileNode(node, createNodeArray(statements, node.statements));
            if (currentModuleInfo.hasExportStarsToExportValues) {
-                setEmitFlags(updated, EmitFlags.EmitExportStar | getEmitFlags(node));
+                addEmitHelper(updated, exportStarHelper);
            }

            return updated;
@@ -110,8 +111,7 @@ namespace ts {
         * @param node The SourceFile node.
         */
        function transformUMDModule(node: SourceFile) {
-            const define = createIdentifier("define");
-            setEmitFlags(define, EmitFlags.UMDDefine);
+            const define = createRawExpression(umdHelper);
            return transformAsynchronousModule(node, define, /*moduleName*/ undefined, /*includeNonAmdDependencies*/ false);
        }

@@ -256,6 +256,7 @@ namespace ts {
            const statementOffset = addPrologueDirectives(statements, node.statements, /*ensureUseStrict*/ !compilerOptions.noImplicitUseStrict, sourceElementVisitor);

            // Visit each statement of the module body.
+            append(statements, visitNode(currentModuleInfo.externalHelpersImportDeclaration, sourceElementVisitor, isStatement, /*optional*/ true));
            addRange(statements, visitNodes(node.statements, sourceElementVisitor, isStatement, statementOffset));

            // End the lexical environment for the module body
@@ -269,7 +270,7 @@ namespace ts {
            if (currentModuleInfo.hasExportStarsToExportValues) {
                // If we have any `export * from ...` declarations
                // we need to inform the emitter to add the __export helper.
-                setEmitFlags(body, EmitFlags.EmitExportStar);
+                addEmitHelper(body, exportStarHelper);
            }

            return body;
@@ -1188,6 +1189,14 @@ namespace ts {
         * @param node The node to substitute.
         */
        function substituteExpressionIdentifier(node: Identifier): Expression {
+            if (getEmitFlags(node) & EmitFlags.HelperName) {
+                const externalHelpersModuleName = getExternalHelpersModuleName(currentSourceFile);
+                if (externalHelpersModuleName) {
+                    return createPropertyAccess(externalHelpersModuleName, node);
+                }
+                return node;
+            }
+
            if (!isGeneratedIdentifier(node) && !isLocalName(node)) {
                const exportContainer = resolver.getReferencedExportContainer(node, isExportName(node));
                if (exportContainer && exportContainer.kind === SyntaxKind.SourceFile) {
@@ -1314,4 +1323,25 @@ namespace ts {
            }
        }
    }
+
+    // emit output for the __export helper function
+    const exportStarHelper: EmitHelper = {
+        name: "typescript:export-star",
+        scoped: true,
+        text: `
+            function __export(m) {
+                for (var p in m) if (!exports.hasOwnProperty(p)) exports[p] = m[p];
+            }`
+    };
+
+    // emit output for the UMD helper function.
+    const umdHelper = `
+        (function (dependencies, factory) {
+            if (typeof module === 'object' && typeof module.exports === 'object') {
+                var v = factory(require, exports); if (v !== undefined) module.exports = v;
+            }
+            else if (typeof define === 'function' && define.amd) {
+                define(dependencies, factory);
+            }
+        })`;
 }
@@ -73,7 +73,7 @@ namespace ts {
            // see comment to 'substitutePostfixUnaryExpression' for more details

            // Collect information about the external module and dependency groups.
-            moduleInfo = moduleInfoMap[id] = collectExternalModuleInfo(node, resolver);
+            moduleInfo = moduleInfoMap[id] = collectExternalModuleInfo(node, resolver, compilerOptions);

            // Make sure that the name of the 'exports' function does not conflict with
            // existing identifiers.
@@ -82,6 +82,7 @@ namespace ts {

            // Add the body of the module.
            const dependencyGroups = collectDependencyGroups(moduleInfo.externalImports);
+            const moduleBodyBlock = createSystemModuleBody(node, dependencyGroups);
            const moduleBodyFunction = createFunctionExpression(
                /*modifiers*/ undefined,
                /*asteriskToken*/ undefined,
@@ -92,7 +93,7 @@ namespace ts {
                    createParameter(/*decorators*/ undefined, /*modifiers*/ undefined, /*dotDotDotToken*/ undefined, contextObject)
                ],
                /*type*/ undefined,
-                createSystemModuleBody(node, dependencyGroups)
+                moduleBodyBlock
            );

            // Write the call to `System.register`
@@ -115,7 +116,9 @@ namespace ts {
                ], node.statements)
            );

-            setEmitFlags(updated, getEmitFlags(node) & ~EmitFlags.EmitEmitHelpers);
+            if (!(compilerOptions.outFile || compilerOptions.out)) {
+                moveEmitHelpers(updated, moduleBodyBlock, helper => !helper.scoped);
+            }

            if (noSubstitution) {
                noSubstitutionMap[id] = noSubstitution;
@@ -236,6 +239,9 @@ namespace ts {
                )
            );

+            // Visit the synthetic external helpers import declaration if present
+            visitNode(moduleInfo.externalHelpersImportDeclaration, sourceElementVisitor, isStatement, /*optional*/ true);
+
            // Visit the statements of the source file, emitting any transformations into
            // the `executeStatements` array. We do this *before* we fill the `setters` array
            // as we both emit transformations as well as aggregate some data used when creating
@@ -280,9 +286,7 @@ namespace ts {
                )
            );

-            const body = createBlock(statements, /*location*/ undefined, /*multiLine*/ true);
-            setEmitFlags(body, EmitFlags.EmitEmitHelpers);
-            return body;
+            return createBlock(statements, /*location*/ undefined, /*multiLine*/ true);
        }

        /**
@@ -394,7 +398,13 @@ namespace ts {
            if (localNames) {
                condition = createLogicalAnd(
                    condition,
-                    createLogicalNot(createHasOwnProperty(localNames, n))
+                    createLogicalNot(
+                        createCall(
+                            createPropertyAccess(localNames, "hasOwnProperty"),
+                            /*typeArguments*/ undefined,
+                            [n]
+                        )
+                    )
                );
            }

@@ -1612,6 +1622,14 @@ namespace ts {
         * @param node The node to substitute.
         */
        function substituteExpressionIdentifier(node: Identifier): Expression {
+            if (getEmitFlags(node) & EmitFlags.HelperName) {
+                const externalHelpersModuleName = getExternalHelpersModuleName(currentSourceFile);
+                if (externalHelpersModuleName) {
+                    return createPropertyAccess(externalHelpersModuleName, node);
+                }
+                return node;
+            }
+
            // When we see an identifier in an expression position that
            // points to an imported symbol, we should substitute a qualified
            // reference to the imported symbol if one is needed.
@@ -49,7 +49,6 @@ namespace ts {
        let currentNamespaceContainerName: Identifier;
        let currentScope: SourceFile | Block | ModuleBlock | CaseBlock;
        let currentScopeFirstDeclarationsOfName: Map<Node>;
-        let currentExternalHelpersModuleName: Identifier;

        /**
         * Keeps track of whether expression substitution has been enabled for specific edge cases.
@@ -81,7 +80,13 @@ namespace ts {
                return node;
            }

-            return visitNode(node, visitor, isSourceFile);
+            currentSourceFile = node;
+
+            const visited = saveStateAndInvoke(node, visitSourceFile);
+            addEmitHelpers(visited, context.readEmitHelpers());
+
+            currentSourceFile = undefined;
+            return visited;
        }

        /**
@@ -108,6 +113,32 @@ namespace ts {
            return visited;
        }

+        /**
+         * Performs actions that should always occur immediately before visiting a node.
+         *
+         * @param node The node to visit.
+         */
+        function onBeforeVisitNode(node: Node) {
+            switch (node.kind) {
+                case SyntaxKind.SourceFile:
+                case SyntaxKind.CaseBlock:
+                case SyntaxKind.ModuleBlock:
+                case SyntaxKind.Block:
+                    currentScope = <SourceFile | CaseBlock | ModuleBlock | Block>node;
+                    currentScopeFirstDeclarationsOfName = undefined;
+                    break;
+
+                case SyntaxKind.ClassDeclaration:
+                case SyntaxKind.FunctionDeclaration:
+                    if (hasModifier(node, ModifierFlags.Ambient)) {
+                        break;
+                    }
+
+                    recordEmittedDeclarationInScope(node);
+                    break;
+            }
+        }
+
        /**
         * General-purpose node visitor.
         *
@@ -123,10 +154,7 @@ namespace ts {
         * @param node The node to visit.
         */
        function visitorWorker(node: Node): VisitResult<Node> {
-            if (node.kind === SyntaxKind.SourceFile) {
-                return visitSourceFile(<SourceFile>node);
-            }
-            else if (node.transformFlags & TransformFlags.TypeScript) {
+            if (node.transformFlags & TransformFlags.TypeScript) {
                // This node is explicitly marked as TypeScript, so we should transform the node.
                return visitTypeScript(node);
            }
@@ -253,7 +281,6 @@ namespace ts {
            return node;
        }

-
        /**
         * Branching visitor, visits a TypeScript syntax node.
         *
@@ -443,78 +470,11 @@ namespace ts {
            }
        }

-        /**
-         * Performs actions that should always occur immediately before visiting a node.
-         *
-         * @param node The node to visit.
-         */
-        function onBeforeVisitNode(node: Node) {
-            switch (node.kind) {
-                case SyntaxKind.SourceFile:
-                case SyntaxKind.CaseBlock:
-                case SyntaxKind.ModuleBlock:
-                case SyntaxKind.Block:
-                    currentScope = <SourceFile | CaseBlock | ModuleBlock | Block>node;
-                    currentScopeFirstDeclarationsOfName = undefined;
-                    break;
-
-                case SyntaxKind.ClassDeclaration:
-                case SyntaxKind.FunctionDeclaration:
-                    if (hasModifier(node, ModifierFlags.Ambient)) {
-                        break;
-                    }
-
-                    recordEmittedDeclarationInScope(node);
-                    break;
-            }
-        }
-
        function visitSourceFile(node: SourceFile) {
-            currentSourceFile = node;
-
-            // ensure "use strict" is emitted in all scenarios in alwaysStrict mode
-            // There is no need to emit "use strict" in the following cases:
-            //      1. The file is an external module and target is es2015 or higher
-            //      or 2. The file is an external module and module-kind is es6 or es2015 as such value is not allowed when targeting es5 or lower
-            if (compilerOptions.alwaysStrict &&
-                !(isExternalModule(node) && (compilerOptions.target >= ScriptTarget.ES2015 || compilerOptions.module === ModuleKind.ES2015))) {
-                node = ensureUseStrict(node);
-            }
-
-            // If the source file requires any helpers and is an external module, and
-            // the importHelpers compiler option is enabled, emit a synthesized import
-            // statement for the helpers library.
-            if (node.flags & NodeFlags.EmitHelperFlags
-                && compilerOptions.importHelpers
-                && (isExternalModule(node) || compilerOptions.isolatedModules)) {
-                startLexicalEnvironment();
-                const statements: Statement[] = [];
-                const statementOffset = addPrologueDirectives(statements, node.statements, /*ensureUseStrict*/ false, visitor);
-                const externalHelpersModuleName = createUniqueName(externalHelpersModuleNameText);
-                const externalHelpersModuleImport = createImportDeclaration(
-                    /*decorators*/ undefined,
-                    /*modifiers*/ undefined,
-                    createImportClause(/*name*/ undefined, createNamespaceImport(externalHelpersModuleName)),
-                    createLiteral(externalHelpersModuleNameText));
-
-                externalHelpersModuleImport.parent = node;
-                externalHelpersModuleImport.flags &= ~NodeFlags.Synthesized;
-                statements.push(externalHelpersModuleImport);
-
-                currentExternalHelpersModuleName = externalHelpersModuleName;
-                addRange(statements, visitNodes(node.statements, sourceElementVisitor, isStatement, statementOffset));
-                addRange(statements, endLexicalEnvironment());
-                currentExternalHelpersModuleName = undefined;
-
-                node = updateSourceFileNode(node, createNodeArray(statements, node.statements));
-                node.externalHelpersModuleName = externalHelpersModuleName;
-            }
-            else {
-                node = visitEachChild(node, sourceElementVisitor, context);
-            }
-
-            setEmitFlags(node, EmitFlags.EmitEmitHelpers | getEmitFlags(node));
-            return node;
+            const alwaysStrict = compilerOptions.alwaysStrict && !(isExternalModule(node) && moduleKind === ModuleKind.ES2015);
+            return updateSourceFileNode(
+                node,
+                visitLexicalEnvironment(node.statements, sourceElementVisitor, context, /*start*/ 0, alwaysStrict));
        }

        /**
@@ -958,15 +918,13 @@ namespace ts {

            // End the lexical environment.
            addRange(statements, endLexicalEnvironment());
-            return setMultiLine(
-                createBlock(
-                    createNodeArray(
-                        statements,
-                        /*location*/ constructor ? constructor.body.statements : node.members
-                    ),
-                    /*location*/ constructor ? constructor.body : /*location*/ undefined
+            return createBlock(
+                createNodeArray(
+                    statements,
+                    /*location*/ constructor ? constructor.body.statements : node.members
                ),
-                true
+                /*location*/ constructor ? constructor.body : /*location*/ undefined,
+                /*multiLine*/ true
            );
        }

@@ -1424,7 +1382,7 @@ namespace ts {
                : undefined;

            const helper = createDecorateHelper(
-                currentExternalHelpersModuleName,
+                context,
                decoratorExpressions,
                prefix,
                memberName,
@@ -1462,7 +1420,7 @@ namespace ts {

            const classAlias = classAliases && classAliases[getOriginalNodeId(node)];
            const localName = getLocalName(node, /*allowComments*/ false, /*allowSourceMaps*/ true);
-            const decorate = createDecorateHelper(currentExternalHelpersModuleName, decoratorExpressions, localName);
+            const decorate = createDecorateHelper(context, decoratorExpressions, localName);
            const expression = createAssignment(localName, classAlias ? createAssignment(classAlias, decorate) : decorate);
            setEmitFlags(expression, EmitFlags.NoComments);
            setSourceMapRange(expression, moveRangePastDecorators(node));
@@ -1490,7 +1448,7 @@ namespace ts {
                expressions = [];
                for (const decorator of decorators) {
                    const helper = createParamHelper(
-                        currentExternalHelpersModuleName,
+                        context,
                        transformDecorator(decorator),
                        parameterOffset,
                        /*location*/ decorator.expression);
@@ -1520,13 +1478,13 @@ namespace ts {
        function addOldTypeMetadata(node: Declaration, decoratorExpressions: Expression[]) {
            if (compilerOptions.emitDecoratorMetadata) {
                if (shouldAddTypeMetadata(node)) {
-                    decoratorExpressions.push(createMetadataHelper(currentExternalHelpersModuleName, "design:type", serializeTypeOfNode(node)));
+                    decoratorExpressions.push(createMetadataHelper(context, "design:type", serializeTypeOfNode(node)));
                }
                if (shouldAddParamTypesMetadata(node)) {
-                    decoratorExpressions.push(createMetadataHelper(currentExternalHelpersModuleName, "design:paramtypes", serializeParameterTypesOfNode(node)));
+                    decoratorExpressions.push(createMetadataHelper(context, "design:paramtypes", serializeParameterTypesOfNode(node)));
                }
                if (shouldAddReturnTypeMetadata(node)) {
-                    decoratorExpressions.push(createMetadataHelper(currentExternalHelpersModuleName, "design:returntype", serializeReturnTypeOfNode(node)));
+                    decoratorExpressions.push(createMetadataHelper(context, "design:returntype", serializeReturnTypeOfNode(node)));
                }
            }
        }
@@ -1544,7 +1502,7 @@ namespace ts {
                    (properties || (properties = [])).push(createPropertyAssignment("returnType", createArrowFunction(/*modifiers*/ undefined, /*typeParameters*/ undefined, [], /*type*/ undefined, createToken(SyntaxKind.EqualsGreaterThanToken), serializeReturnTypeOfNode(node))));
                }
                if (properties) {
-                    decoratorExpressions.push(createMetadataHelper(currentExternalHelpersModuleName, "design:typeinfo", createObjectLiteral(properties, /*location*/ undefined, /*multiLine*/ true)));
+                    decoratorExpressions.push(createMetadataHelper(context, "design:typeinfo", createObjectLiteral(properties, /*location*/ undefined, /*multiLine*/ true)));
                }
            }
        }
@@ -3286,7 +3244,7 @@ namespace ts {

        function trySubstituteNamespaceExportedName(node: Identifier): Expression {
            // If this is explicitly a local name, do not substitute.
-            if (enabledSubstitutions & applicableSubstitutions && !isLocalName(node)) {
+            if (enabledSubstitutions & applicableSubstitutions && !isGeneratedIdentifier(node) && !isLocalName(node)) {
                // If we are nested within a namespace declaration, we may need to qualifiy
                // an identifier that is exported from a merged namespace.
                const container = resolver.getReferencedExportContainer(node, /*prefixLocals*/ false);
@@ -3341,4 +3299,77 @@ namespace ts {
                : undefined;
        }
    }
+
+    const paramHelper: EmitHelper = {
+        name: "typescript:param",
+        scoped: false,
+        priority: 4,
+        text: `
+            var __param = (this && this.__param) || function (paramIndex, decorator) {
+                return function (target, key) { decorator(target, key, paramIndex); }
+            };`
+    };
+
+    function createParamHelper(context: TransformationContext, expression: Expression, parameterOffset: number, location?: TextRange) {
+        context.requestEmitHelper(paramHelper);
+        return createCall(
+            getHelperName("__param"),
+            /*typeArguments*/ undefined,
+            [
+                createLiteral(parameterOffset),
+                expression
+            ],
+            location
+        );
+    }
+
+    const metadataHelper: EmitHelper = {
+        name: "typescript:metadata",
+        scoped: false,
+        priority: 3,
+        text: `
+            var __metadata = (this && this.__metadata) || function (k, v) {
+                if (typeof Reflect === "object" && typeof Reflect.metadata === "function") return Reflect.metadata(k, v);
+            };`
+    };
+
+    function createMetadataHelper(context: TransformationContext, metadataKey: string, metadataValue: Expression) {
+        context.requestEmitHelper(metadataHelper);
+        return createCall(
+            getHelperName("__metadata"),
+            /*typeArguments*/ undefined,
+            [
+                createLiteral(metadataKey),
+                metadataValue
+            ]
+        );
+    }
+
+    const decorateHelper: EmitHelper = {
+        name: "typescript:decorate",
+        scoped: false,
+        priority: 2,
+        text: `
+            var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
+                var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
+                if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
+                else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
+                return c > 3 && r && Object.defineProperty(target, key, r), r;
+            };`
+    };
+
+    function createDecorateHelper(context: TransformationContext, decoratorExpressions: Expression[], target: Expression, memberName?: Expression, descriptor?: Expression, location?: TextRange) {
+        context.requestEmitHelper(decorateHelper);
+        const argumentsArray: Expression[] = [];
+        argumentsArray.push(createArrayLiteral(decoratorExpressions, /*location*/ undefined, /*multiLine*/ true));
+        argumentsArray.push(target);
+        if (memberName) {
+            argumentsArray.push(memberName);
+            if (descriptor) {
+                argumentsArray.push(descriptor);
+            }
+        }
+
+        return createCall(getHelperName("__decorate"), /*typeArguments*/ undefined, argumentsArray, location);
+    }
 }
@@ -369,6 +369,7 @@ namespace ts {
        PartiallyEmittedExpression,
        MergeDeclarationMarker,
        EndOfDeclarationMarker,
+        RawExpression,

        // Enum value count
        Count,
@@ -1523,6 +1524,16 @@ namespace ts {
        kind: SyntaxKind.EndOfDeclarationMarker;
    }

+    /**
+     * Emits a string of raw text in an expression position. Raw text is never transformed, should
+     * be ES3 compliant, and should have the same precedence as PrimaryExpression.
+     */
+    /* @internal */
+    export interface RawExpression extends PrimaryExpression {
+        kind: SyntaxKind.RawExpression;
+        text: string;
+    }
+
    /**
     * Marks the beginning of a merged transformed declaration.
     */
@@ -1562,6 +1573,11 @@ namespace ts {
        expression: Expression;
    }

+    /* @internal */
+    export interface PrologueDirective extends ExpressionStatement {
+        expression: StringLiteral;
+    }
+
    export interface IfStatement extends Statement {
        kind: SyntaxKind.IfStatement;
        expression: Expression;
@@ -2162,8 +2178,6 @@ namespace ts {
        /* @internal */ moduleAugmentations: LiteralExpression[];
        /* @internal */ patternAmbientModules?: PatternAmbientModule[];
        /* @internal */ ambientModuleNames: string[];
-        // The synthesized identifier for an imported external helpers module.
-        /* @internal */ externalHelpersModuleName?: Identifier;
    }

    export interface ScriptReferenceHost {
@@ -3626,45 +3640,51 @@ namespace ts {

    /* @internal */
    export interface EmitNode {
-        flags?: EmitFlags;
-        commentRange?: TextRange;
-        sourceMapRange?: TextRange;
-        tokenSourceMapRanges?: Map<TextRange>;
        annotatedNodes?: Node[];                // Tracks Parse-tree nodes with EmitNodes for eventual cleanup.
-        constantValue?: number;
+        flags?: EmitFlags;                      // Flags that customize emit
+        commentRange?: TextRange;               // The text range to use when emitting leading or trailing comments
+        sourceMapRange?: TextRange;             // The text range to use when emitting leading or trailing source mappings
+        tokenSourceMapRanges?: Map<TextRange>;  // The text range to use when emitting source mappings for tokens
+        constantValue?: number;                 // The constant value of an expression
+        externalHelpersModuleName?: Identifier; // The local name for an imported helpers module
+        helpers?: EmitHelper[];                 // Emit helpers for the node
    }

    /* @internal */
    export const enum EmitFlags {
-        EmitEmitHelpers = 1 << 0,                // Any emit helpers should be written to this node.
-        EmitExportStar = 1 << 1,                 // The export * helper should be written to this node.
-        EmitSuperHelper = 1 << 2,                // Emit the basic _super helper for async methods.
-        EmitAdvancedSuperHelper = 1 << 3,        // Emit the advanced _super helper for async methods.
-        UMDDefine = 1 << 4,                      // This node should be replaced with the UMD define helper.
-        SingleLine = 1 << 5,                     // The contents of this node should be emitted on a single line.
-        AdviseOnEmitNode = 1 << 6,               // The printer should invoke the onEmitNode callback when printing this node.
-        NoSubstitution = 1 << 7,                 // Disables further substitution of an expression.
-        CapturesThis = 1 << 8,                   // The function captures a lexical `this`
-        NoLeadingSourceMap = 1 << 9,             // Do not emit a leading source map location for this node.
-        NoTrailingSourceMap = 1 << 10,           // Do not emit a trailing source map location for this node.
+        SingleLine = 1 << 0,                     // The contents of this node should be emitted on a single line.
+        AdviseOnEmitNode = 1 << 1,               // The printer should invoke the onEmitNode callback when printing this node.
+        NoSubstitution = 1 << 2,                 // Disables further substitution of an expression.
+        CapturesThis = 1 << 3,                   // The function captures a lexical `this`
+        NoLeadingSourceMap = 1 << 4,             // Do not emit a leading source map location for this node.
+        NoTrailingSourceMap = 1 << 5,            // Do not emit a trailing source map location for this node.
        NoSourceMap = NoLeadingSourceMap | NoTrailingSourceMap, // Do not emit a source map location for this node.
-        NoNestedSourceMaps = 1 << 11,            // Do not emit source map locations for children of this node.
-        NoTokenLeadingSourceMaps = 1 << 12,      // Do not emit leading source map location for token nodes.
-        NoTokenTrailingSourceMaps = 1 << 13,     // Do not emit trailing source map location for token nodes.
+        NoNestedSourceMaps = 1 << 6,             // Do not emit source map locations for children of this node.
+        NoTokenLeadingSourceMaps = 1 << 7,       // Do not emit leading source map location for token nodes.
+        NoTokenTrailingSourceMaps = 1 << 8,      // Do not emit trailing source map location for token nodes.
        NoTokenSourceMaps = NoTokenLeadingSourceMaps | NoTokenTrailingSourceMaps, // Do not emit source map locations for tokens of this node.
-        NoLeadingComments = 1 << 14,             // Do not emit leading comments for this node.
-        NoTrailingComments = 1 << 15,            // Do not emit trailing comments for this node.
+        NoLeadingComments = 1 << 9,              // Do not emit leading comments for this node.
+        NoTrailingComments = 1 << 10,            // Do not emit trailing comments for this node.
        NoComments = NoLeadingComments | NoTrailingComments, // Do not emit comments for this node.
-        NoNestedComments = 1 << 16,
-        ExportName = 1 << 17,                    // Ensure an export prefix is added for an identifier that points to an exported declaration with a local name (see SymbolFlags.ExportHasLocal).
-        LocalName = 1 << 18,                     // Ensure an export prefix is not added for an identifier that points to an exported declaration.
-        Indented = 1 << 19,                      // Adds an explicit extra indentation level for class and function bodies when printing (used to match old emitter).
-        NoIndentation = 1 << 20,                 // Do not indent the node.
-        AsyncFunctionBody = 1 << 21,
-        ReuseTempVariableScope = 1 << 22,        // Reuse the existing temp variable scope during emit.
-        CustomPrologue = 1 << 23,                // Treat the statement as if it were a prologue directive (NOTE: Prologue directives are *not* transformed).
-        NoHoisting = 1 << 24,                    // Do not hoist this declaration in --module system
-        HasEndOfDeclarationMarker = 1 << 25,     // Declaration has an associated NotEmittedStatement to mark the end of the declaration
+        NoNestedComments = 1 << 11,
+        HelperName = 1 << 12,
+        ExportName = 1 << 13,                    // Ensure an export prefix is added for an identifier that points to an exported declaration with a local name (see SymbolFlags.ExportHasLocal).
+        LocalName = 1 << 14,                     // Ensure an export prefix is not added for an identifier that points to an exported declaration.
+        Indented = 1 << 15,                      // Adds an explicit extra indentation level for class and function bodies when printing (used to match old emitter).
+        NoIndentation = 1 << 16,                 // Do not indent the node.
+        AsyncFunctionBody = 1 << 17,
+        ReuseTempVariableScope = 1 << 18,        // Reuse the existing temp variable scope during emit.
+        CustomPrologue = 1 << 19,                // Treat the statement as if it were a prologue directive (NOTE: Prologue directives are *not* transformed).
+        NoHoisting = 1 << 20,                    // Do not hoist this declaration in --module system
+        HasEndOfDeclarationMarker = 1 << 21,     // Declaration has an associated NotEmittedStatement to mark the end of the declaration
+    }
+
+    /* @internal */
+    export interface EmitHelper {
+        readonly name: string;      // A unique name for this helper.
+        readonly scoped: boolean;   // Indicates whether ther helper MUST be emitted in the current scope.
+        readonly text: string;      // ES3-compatible raw script text.
+        readonly priority?: number; // Helpers with a higher priority are emitted earlier than other helpers on the node.
    }

    /* @internal */
@@ -3675,19 +3695,124 @@ namespace ts {
        Unspecified,        // Emitting an otherwise unspecified node
    }

-    /** Additional context provided to `visitEachChild` */
    /* @internal */
-    export interface LexicalEnvironment {
+    export interface EmitHost extends ScriptReferenceHost {
+        getSourceFiles(): SourceFile[];
+
+        /* @internal */
+        isSourceFileFromExternalLibrary(file: SourceFile): boolean;
+
+        getCommonSourceDirectory(): string;
+        getCanonicalFileName(fileName: string): string;
+        getNewLine(): string;
+
+        isEmitBlocked(emitFileName: string): boolean;
+
+        writeFile: WriteFileCallback;
+    }
+
+    /* @internal */
+    export interface TransformationContext {
+        getCompilerOptions(): CompilerOptions;
+        getEmitResolver(): EmitResolver;
+        getEmitHost(): EmitHost;
+
        /** Starts a new lexical environment. */
        startLexicalEnvironment(): void;

+        /** Suspends the current lexical environment, usually after visiting a parameter list. */
        suspendLexicalEnvironment(): void;
+
+        /** Resumes a suspended lexical environment, usually before visiting a function body. */
        resumeLexicalEnvironment(): void;

        /** Ends a lexical environment, returning any declarations. */
        endLexicalEnvironment(): Statement[];
+
+        /**
+         * Hoists a function declaration to the containing scope.
+         */
+        hoistFunctionDeclaration(node: FunctionDeclaration): void;
+
+        /**
+         * Hoists a variable declaration to the containing scope.
+         */
+        hoistVariableDeclaration(node: Identifier): void;
+
+        /**
+         * Records a request for a non-scoped emit helper in the current context.
+         */
+        requestEmitHelper(helper: EmitHelper): void;
+
+        /**
+         * Gets and resets the requested non-scoped emit helpers.
+         */
+        readEmitHelpers(): EmitHelper[] | undefined;
+
+        /**
+         * Enables expression substitutions in the pretty printer for the provided SyntaxKind.
+         */
+        enableSubstitution(kind: SyntaxKind): void;
+
+        /**
+         * Determines whether expression substitutions are enabled for the provided node.
+         */
+        isSubstitutionEnabled(node: Node): boolean;
+
+        /**
+         * Hook used by transformers to substitute expressions just before they
+         * are emitted by the pretty printer.
+         */
+        onSubstituteNode?: (emitContext: EmitContext, node: Node) => Node;
+
+        /**
+         * Enables before/after emit notifications in the pretty printer for the provided
+         * SyntaxKind.
+         */
+        enableEmitNotification(kind: SyntaxKind): void;
+
+        /**
+         * Determines whether before/after emit notifications should be raised in the pretty
+         * printer when it emits a node.
+         */
+        isEmitNotificationEnabled(node: Node): boolean;
+
+        /**
+         * Hook used to allow transformers to capture state before or after
+         * the printer emits a node.
+         */
+        onEmitNode?: (emitContext: EmitContext, node: Node, emitCallback: (emitContext: EmitContext, node: Node) => void) => void;
    }

+    /* @internal */
+    export interface TransformationResult {
+        /**
+         * Gets the transformed source files.
+         */
+        transformed: SourceFile[];
+
+        /**
+         * Emits the substitute for a node, if one is available; otherwise, emits the node.
+         *
+         * @param emitContext The current emit context.
+         * @param node The node to substitute.
+         * @param emitCallback A callback used to emit the node or its substitute.
+         */
+        emitNodeWithSubstitution(emitContext: EmitContext, node: Node, emitCallback: (emitContext: EmitContext, node: Node) => void): void;
+
+        /**
+         * Emits a node with possible notification.
+         *
+         * @param emitContext The current emit context.
+         * @param node The node to emit.
+         * @param emitCallback A callback used to emit the node.
+         */
+        emitNodeWithNotification(emitContext: EmitContext, node: Node, emitCallback: (emitContext: EmitContext, node: Node) => void): void;
+    }
+
+    /* @internal */
+    export type Transformer = (context: TransformationContext) => (node: SourceFile) => SourceFile;
+
    export interface TextSpan {
        start: number;
        length: number;
@@ -28,21 +28,6 @@ namespace ts {
        string(): string;
    }

-    export interface EmitHost extends ScriptReferenceHost {
-        getSourceFiles(): SourceFile[];
-
-        /* @internal */
-        isSourceFileFromExternalLibrary(file: SourceFile): boolean;
-
-        getCommonSourceDirectory(): string;
-        getCanonicalFileName(fileName: string): string;
-        getNewLine(): string;
-
-        isEmitBlocked(emitFileName: string): boolean;
-
-        writeFile: WriteFileCallback;
-    }
-
    // Pool writers to avoid needing to allocate them for every symbol we write.
    const stringWriters: StringSymbolWriter[] = [];
    export function getSingleLineStringWriter(): StringSymbolWriter {
@@ -626,8 +611,9 @@ namespace ts {
        return n.kind === SyntaxKind.CallExpression && (<CallExpression>n).expression.kind === SyntaxKind.SuperKeyword;
    }

-    export function isPrologueDirective(node: Node): boolean {
-        return node.kind === SyntaxKind.ExpressionStatement && (<ExpressionStatement>node).expression.kind === SyntaxKind.StringLiteral;
+    export function isPrologueDirective(node: Node): node is PrologueDirective {
+        return node.kind === SyntaxKind.ExpressionStatement
+            && (<ExpressionStatement>node).expression.kind === SyntaxKind.StringLiteral;
    }

    export function getLeadingCommentRangesOfNode(node: Node, sourceFileOfNode: SourceFile) {
@@ -2148,6 +2134,7 @@ namespace ts {
            case SyntaxKind.TemplateExpression:
            case SyntaxKind.ParenthesizedExpression:
            case SyntaxKind.OmittedExpression:
+            case SyntaxKind.RawExpression:
                return 19;

            case SyntaxKind.TaggedTemplateExpression:
@@ -3572,153 +3559,6 @@ namespace ts {
        return positionIsSynthesized(range.pos) ? -1 : skipTrivia(sourceFile.text, range.pos);
    }

-    export interface ExternalModuleInfo {
-        externalImports: (ImportDeclaration | ImportEqualsDeclaration | ExportDeclaration)[]; // imports of other external modules
-        exportSpecifiers: Map<ExportSpecifier[]>; // export specifiers by name
-        exportedBindings: Map<Identifier[]>; // exported names of local declarations
-        exportedNames: Identifier[]; // all exported names local to module
-        exportEquals: ExportAssignment | undefined; // an export= declaration if one was present
-        hasExportStarsToExportValues: boolean; // whether this module contains export*
-    }
-
-    export function collectExternalModuleInfo(sourceFile: SourceFile, resolver: EmitResolver): ExternalModuleInfo {
-        const externalImports: (ImportDeclaration | ImportEqualsDeclaration | ExportDeclaration)[] = [];
-        const exportSpecifiers = createMap<ExportSpecifier[]>();
-        const exportedBindings = createMap<Identifier[]>();
-        const uniqueExports = createMap<Identifier>();
-        let hasExportDefault = false;
-        let exportEquals: ExportAssignment = undefined;
-        let hasExportStarsToExportValues = false;
-        for (const node of sourceFile.statements) {
-            switch (node.kind) {
-                case SyntaxKind.ImportDeclaration:
-                    // import "mod"
-                    // import x from "mod"
-                    // import * as x from "mod"
-                    // import { x, y } from "mod"
-                    externalImports.push(<ImportDeclaration>node);
-                    break;
-
-                case SyntaxKind.ImportEqualsDeclaration:
-                    if ((<ImportEqualsDeclaration>node).moduleReference.kind === SyntaxKind.ExternalModuleReference) {
-                        // import x = require("mod")
-                        externalImports.push(<ImportEqualsDeclaration>node);
-                    }
-
-                    break;
-
-                case SyntaxKind.ExportDeclaration:
-                    if ((<ExportDeclaration>node).moduleSpecifier) {
-                        if (!(<ExportDeclaration>node).exportClause) {
-                            // export * from "mod"
-                            externalImports.push(<ExportDeclaration>node);
-                            hasExportStarsToExportValues = true;
-                        }
-                        else {
-                            // export { x, y } from "mod"
-                            externalImports.push(<ExportDeclaration>node);
-                        }
-                    }
-                    else {
-                        // export { x, y }
-                        for (const specifier of (<ExportDeclaration>node).exportClause.elements) {
-                            if (!uniqueExports[specifier.name.text]) {
-                                const name = specifier.propertyName || specifier.name;
-                                multiMapAdd(exportSpecifiers, name.text, specifier);
-
-                                const decl = resolver.getReferencedImportDeclaration(name)
-                                    || resolver.getReferencedValueDeclaration(name);
-
-                                if (decl) {
-                                    multiMapAdd(exportedBindings, getOriginalNodeId(decl), specifier.name);
-                                }
-
-                                uniqueExports[specifier.name.text] = specifier.name;
-                            }
-                        }
-                    }
-                    break;
-
-                case SyntaxKind.ExportAssignment:
-                    if ((<ExportAssignment>node).isExportEquals && !exportEquals) {
-                        // export = x
-                        exportEquals = <ExportAssignment>node;
-                    }
-                    break;
-
-                case SyntaxKind.VariableStatement:
-                    if (hasModifier(node, ModifierFlags.Export)) {
-                        for (const decl of (<VariableStatement>node).declarationList.declarations) {
-                            collectExportedVariableInfo(decl, uniqueExports);
-                        }
-                    }
-                    break;
-
-                case SyntaxKind.FunctionDeclaration:
-                    if (hasModifier(node, ModifierFlags.Export)) {
-                        if (hasModifier(node, ModifierFlags.Default)) {
-                            // export default function() { }
-                            if (!hasExportDefault) {
-                                multiMapAdd(exportedBindings, getOriginalNodeId(node), getDeclarationName(<FunctionDeclaration>node));
-                                hasExportDefault = true;
-                            }
-                        }
-                        else {
-                            // export function x() { }
-                            const name = (<FunctionDeclaration>node).name;
-                            if (!uniqueExports[name.text]) {
-                                multiMapAdd(exportedBindings, getOriginalNodeId(node), name);
-                                uniqueExports[name.text] = name;
-                            }
-                        }
-                    }
-                    break;
-
-                case SyntaxKind.ClassDeclaration:
-                    if (hasModifier(node, ModifierFlags.Export)) {
-                        if (hasModifier(node, ModifierFlags.Default)) {
-                            // export default class { }
-                            if (!hasExportDefault) {
-                                multiMapAdd(exportedBindings, getOriginalNodeId(node), getDeclarationName(<ClassDeclaration>node));
-                                hasExportDefault = true;
-                            }
-                        }
-                        else {
-                            // export class x { }
-                            const name = (<ClassDeclaration>node).name;
-                            if (!uniqueExports[name.text]) {
-                                multiMapAdd(exportedBindings, getOriginalNodeId(node), name);
-                                uniqueExports[name.text] = name;
-                            }
-                        }
-                    }
-                    break;
-            }
-        }
-
-        let exportedNames: Identifier[];
-        for (const key in uniqueExports) {
-            exportedNames = ts.append(exportedNames, uniqueExports[key]);
-        }
-
-        return { externalImports, exportSpecifiers, exportEquals, hasExportStarsToExportValues, exportedBindings, exportedNames };
-    }
-
-    function collectExportedVariableInfo(decl: VariableDeclaration | BindingElement, uniqueExports: Map<Identifier>) {
-        if (isBindingPattern(decl.name)) {
-            for (const element of decl.name.elements) {
-                if (!isOmittedExpression(element)) {
-                    collectExportedVariableInfo(element, uniqueExports);
-                }
-            }
-        }
-        else if (!isGeneratedIdentifier(decl.name)) {
-            if (!uniqueExports[decl.name.text]) {
-                uniqueExports[decl.name.text] = decl.name;
-            }
-        }
-    }
-
    /**
     * Determines whether a name was originally the declaration name of an enum or namespace
     * declaration.
@@ -4086,7 +3926,8 @@ namespace ts {
            || kind === SyntaxKind.ThisKeyword
            || kind === SyntaxKind.TrueKeyword
            || kind === SyntaxKind.SuperKeyword
-            || kind === SyntaxKind.NonNullExpression;
+            || kind === SyntaxKind.NonNullExpression
+            || kind === SyntaxKind.RawExpression;
    }

    export function isLeftHandSideExpression(node: Node): node is LeftHandSideExpression {
@@ -4116,6 +3957,7 @@ namespace ts {
            || kind === SyntaxKind.SpreadElement
            || kind === SyntaxKind.AsExpression
            || kind === SyntaxKind.OmittedExpression
+            || kind === SyntaxKind.RawExpression
            || isUnaryExpressionKind(kind);
    }

@@ -669,9 +669,12 @@ namespace ts {
     * Starts a new lexical environment and visits a statement list, ending the lexical environment
     * and merging hoisted declarations upon completion.
     */
-    export function visitLexicalEnvironment(statements: NodeArray<Statement>, visitor: (node: Node) => VisitResult<Node>, context: LexicalEnvironment, start?: number) {
+    export function visitLexicalEnvironment(statements: NodeArray<Statement>, visitor: (node: Node) => VisitResult<Node>, context: TransformationContext, start?: number, ensureUseStrict?: boolean) {
        context.startLexicalEnvironment();
        statements = visitNodes(statements, visitor, isStatement, start);
+        if (ensureUseStrict && !startsWithUseStrict(statements)) {
+            statements = createNodeArray([createStatement(createLiteral("use strict")), ...statements], statements);
+        }
        const declarations = context.endLexicalEnvironment();
        return createNodeArray(concatenate(statements, declarations), statements);
    }
@@ -680,9 +683,9 @@ namespace ts {
     * Starts a new lexical environment and visits a parameter list, suspending the lexical
     * environment upon completion.
     */
-    export function visitParameterList(nodes: NodeArray<ParameterDeclaration>, visitor: (node: Node) => VisitResult<Node>, context: LexicalEnvironment) {
+    export function visitParameterList(nodes: NodeArray<ParameterDeclaration>, visitor: (node: Node) => VisitResult<Node>, context: TransformationContext) {
        context.startLexicalEnvironment();
-        const updated = visitNodes(nodes, visitor, isParameter);
+        const updated = visitNodes(nodes, visitor, isParameterDeclaration);
        context.suspendLexicalEnvironment();
        return updated;
    }
@@ -691,21 +694,22 @@ namespace ts {
     * Resumes a suspended lexical environment and visits a function body, ending the lexical
     * environment and merging hoisted declarations upon completion.
     */
-    export function visitFunctionBody(node: FunctionBody, visitor: (node: Node) => VisitResult<Node>, context: LexicalEnvironment): FunctionBody;
+    export function visitFunctionBody(node: FunctionBody, visitor: (node: Node) => VisitResult<Node>, context: TransformationContext): FunctionBody;
    /**
     * Resumes a suspended lexical environment and visits a concise body, ending the lexical
     * environment and merging hoisted declarations upon completion.
     */
-    export function visitFunctionBody(node: ConciseBody, visitor: (node: Node) => VisitResult<Node>, context: LexicalEnvironment): ConciseBody;
-    export function visitFunctionBody(node: ConciseBody, visitor: (node: Node) => VisitResult<Node>, context: LexicalEnvironment) {
+    export function visitFunctionBody(node: ConciseBody, visitor: (node: Node) => VisitResult<Node>, context: TransformationContext): ConciseBody;
+    export function visitFunctionBody(node: ConciseBody, visitor: (node: Node) => VisitResult<Node>, context: TransformationContext): ConciseBody {
        context.resumeLexicalEnvironment();
-        const visited = visitNode(node, visitor, isConciseBody);
+        const updated = visitNode(node, visitor, isConciseBody);
        const declarations = context.endLexicalEnvironment();
        if (some(declarations)) {
-            const block = convertToFunctionBody(visited);
-            return updateBlock(block, createNodeArray(concatenate(block.statements, declarations), block.statements));
+            const block = convertToFunctionBody(updated);
+            const statements = mergeLexicalEnvironment(block.statements, declarations);
+            return updateBlock(block, statements);
        }
-        return visited;
+        return updated;
    }

    /**
@@ -715,8 +719,8 @@ namespace ts {
     * @param visitor The callback used to visit each child.
     * @param context A lexical environment context for the visitor.
     */
-    export function visitEachChild<T extends Node>(node: T, visitor: (node: Node) => VisitResult<Node>, context: LexicalEnvironment): T;
-    export function visitEachChild(node: Node, visitor: (node: Node) => VisitResult<Node>, context: LexicalEnvironment): Node {
+    export function visitEachChild<T extends Node>(node: T, visitor: (node: Node) => VisitResult<Node>, context: TransformationContext): T;
+    export function visitEachChild(node: Node, visitor: (node: Node) => VisitResult<Node>, context: TransformationContext): Node {
        if (node === undefined) {
            return undefined;
        }
@@ -1042,7 +1046,7 @@ namespace ts {
                    visitNodes((<FunctionDeclaration>node).typeParameters, visitor, isTypeParameter),
                    visitParameterList((<FunctionDeclaration>node).parameters, visitor, context),
                    visitNode((<FunctionDeclaration>node).type, visitor, isTypeNode, /*optional*/ true),
-                    visitFunctionBody((<FunctionDeclaration>node).body, visitor, context));
+                    visitFunctionBody((<FunctionExpression>node).body, visitor, context));

            case SyntaxKind.ClassDeclaration:
                return updateClassDeclaration(<ClassDeclaration>node,
@@ -1209,6 +1213,24 @@ namespace ts {
        // return node;
    }

+    /**
+     * Merges generated lexical declarations into a new statement list.
+     */
+    export function mergeLexicalEnvironment(statements: NodeArray<Statement>, declarations: Statement[]): NodeArray<Statement>;
+    /**
+     * Appends generated lexical declarations to an array of statements.
+     */
+    export function mergeLexicalEnvironment(statements: Statement[], declarations: Statement[]): Statement[];
+    export function mergeLexicalEnvironment(statements: Statement[], declarations: Statement[]) {
+        if (!some(declarations)) {
+            return statements;
+        }
+        return isNodeArray(statements)
+            ? createNodeArray(concatenate(statements, declarations), statements)
+            : addRange(statements, declarations);
+    }
+
+
    /**
     * Merges generated lexical declarations into the FunctionBody of a non-arrow function-like declaration.
     *
@@ -67,8 +67,8 @@ C = tslib_1.__decorate([
    tslib_1.__metadata("design:paramtypes", [])
 ], C);
 var o = { a: 1 };
-var y = __assign({}, o);
-var x = __rest(y, []);
+var y = tslib_1.__assign({}, o);
+var x = tslib_1.__rest(y, []);
 //// [script.js]
 var __extends = (this && this.__extends) || function (d, b) {
    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];