Added test for noCustomAsyncPromise switch

tests/baselines/reference/noCustomAsyncReturnType_es6.errors.txt

@@ -0,0 +1,16 @@
+tests/cases/conformance/async/es6/noCustomAsyncReturnType_es6.ts(3,5): error TS2529: Duplicate identifier 'Promise'. Compiler reserves name 'Promise' in top level scope of a module containing async functions.
+tests/cases/conformance/async/es6/noCustomAsyncReturnType_es6.ts(5,28): error TS1064: The return type of an async function or method must be the global Promise<T> type.
+
+
+==== tests/cases/conformance/async/es6/noCustomAsyncReturnType_es6.ts (2 errors) ====
+    
+    interface Promise<T> extends PromiseLike<T> {}
+    var Promise: PromiseConstructorLike;
+        ~~~~~~~
+!!! error TS2529: Duplicate identifier 'Promise'. Compiler reserves name 'Promise' in top level scope of a module containing async functions.
+    
+    export async function f(): Promise<void> {
+                               ~~~~~~~~~~~~~
+!!! error TS1064: The return type of an async function or method must be the global Promise<T> type.
+    }
+    

tests/baselines/reference/noCustomAsyncReturnType_es6.js

@@ -0,0 +1,23 @@
+//// [noCustomAsyncReturnType_es6.ts]
+
+interface Promise<T> extends PromiseLike<T> {}
+var Promise: PromiseConstructorLike;
+
+export async function f(): Promise<void> {
+}
+
+
+//// [noCustomAsyncReturnType_es6.js]
+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());
+    });
+};
+var Promise;
+export function f() {
+    return __awaiter(this, void 0, void 0, function* () {
+    });
+}

tests/cases/conformance/async/es6/noCustomAsyncReturnType_es6.ts

@@ -0,0 +1,8 @@
+// @target: ES6
+// @noCustomAsyncPromise: true
+
+interface Promise<T> extends PromiseLike<T> {}
+var Promise: PromiseConstructorLike;
+
+export async function f(): Promise<void> {
+}