diff --git a/flow/babel.js.flow b/flow/babel.js.flow
index 372b4ea0755..ad97546cc12 100644
--- a/flow/babel.js.flow
+++ b/flow/babel.js.flow
@@ -9,11 +9,11 @@
 
 type _SourceMap = {
   version: number,
-  file: string,
+  file?: string,
   sources: Array<string>,
   names: Array<string>,
   mappings: string,
-  sourcesContent: Array<string>,
+  sourcesContent?: Array<string>,
 };
 
 // based on babylon v6.13.1
diff --git a/local-cli/bundle/buildBundle.js b/local-cli/bundle/buildBundle.js
index 0a4d0282816..a8b3c44eacc 100644
--- a/local-cli/bundle/buildBundle.js
+++ b/local-cli/bundle/buildBundle.js
@@ -16,6 +16,8 @@ const path = require('path');
 const saveAssets = require('./saveAssets');
 const defaultAssetExts = require('../../packager/defaults').assetExts;
 
+import type RequestOptions from './types.flow';
+
 function saveBundle(output, bundle, args) {
   return Promise.resolve(
     output.save(bundle, args, log)
@@ -27,7 +29,7 @@ function buildBundle(args, config, output = outputBundle, packagerInstance) {
   // have other choice than defining it as an env variable here.
   process.env.NODE_ENV = args.dev ? 'development' : 'production';
 
-  const requestOpts = {
+  const requestOpts: RequestOptions = {
     entryFile: args.entryFile,
     sourceMapUrl: args.sourcemapOutput && path.basename(args.sourcemapOutput),
     dev: args.dev,
diff --git a/local-cli/bundle/output/bundle.js b/local-cli/bundle/output/bundle.js
index a54ebe9de85..06217ba3061 100644
--- a/local-cli/bundle/output/bundle.js
+++ b/local-cli/bundle/output/bundle.js
@@ -5,27 +5,37 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
  */
 'use strict';
 
 const meta = require('./meta');
 const writeFile = require('./writeFile');
 
-function buildBundle(packagerClient, requestOptions) {
+import type Bundle from '../../../packager/react-packager/src/Bundler/Bundle';
+import type Server from '../../../packager/react-packager/src/Server';
+import type {OutputOptions, RequestOptions} from '../types.flow';
+
+function buildBundle(packagerClient: Server, requestOptions: RequestOptions) {
   return packagerClient.buildBundle({
     ...requestOptions,
     isolateModuleIDs: true,
   });
 }
 
-function createCodeWithMap(bundle, dev) {
+function createCodeWithMap(bundle: Bundle, dev: boolean): * {
   return {
     code: bundle.getSource({dev}),
     map: JSON.stringify(bundle.getSourceMap({dev})),
   };
 }
 
-function saveBundleAndMap(bundle, options, log) {
+function saveBundleAndMap(
+  bundle: Bundle,
+  options: OutputOptions,
+  log: (x: string) => {},
+): Promise<> {
   const {
     bundleOutput,
     bundleEncoding: encoding,
@@ -34,7 +44,7 @@ function saveBundleAndMap(bundle, options, log) {
   } = options;
 
   log('start');
-  const codeWithMap = createCodeWithMap(bundle, dev);
+  const codeWithMap = createCodeWithMap(bundle, !!dev);
   log('finish');
 
   log('Writing bundle output to:', bundleOutput);
diff --git a/local-cli/bundle/output/unbundle/as-assets.js b/local-cli/bundle/output/unbundle/as-assets.js
index 82258502cb5..ae2d8c1d813 100644
--- a/local-cli/bundle/output/unbundle/as-assets.js
+++ b/local-cli/bundle/output/unbundle/as-assets.js
@@ -5,6 +5,8 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
  */
 'use strict';
 
@@ -18,6 +20,9 @@ const writeSourceMap = require('./write-sourcemap');
 
 const {joinModules} = require('./util');
 
+import type Bundle from '../../../../packager/react-packager/src/Bundler/Bundle';
+import type {OutputOptions} from '../../types.flow';
+
 // must not start with a dot, as that won't go into the apk
 const MAGIC_UNBUNDLE_FILENAME = 'UNBUNDLE';
 const MODULES_DIR = 'js-modules';
@@ -29,7 +34,11 @@ const MODULES_DIR = 'js-modules';
  * All other modules go into a 'js-modules' folder that in the same parent
  * directory as the startup file.
  */
-function saveAsAssets(bundle, options, log) {
+function saveAsAssets(
+  bundle: Bundle,
+  options: OutputOptions,
+  log: (x: string) => void,
+): Promise<mixed> {
   const {
     bundleOutput,
     bundleEncoding: encoding,
@@ -54,11 +63,14 @@ function saveAsAssets(bundle, options, log) {
   writeUnbundle.then(() => log('Done writing unbundle output'));
 
   const sourceMap =
-    buildSourceMapWithMetaData({startupModules, lazyModules});
+    buildSourceMapWithMetaData({
+      startupModules: startupModules.concat(),
+      lazyModules: lazyModules.concat(),
+    });
 
   return Promise.all([
     writeUnbundle,
-    writeSourceMap(sourcemapOutput, JSON.stringify(sourceMap), log)
+    sourcemapOutput && writeSourceMap(sourcemapOutput, JSON.stringify(sourceMap), log)
   ]);
 }
 
@@ -80,8 +92,8 @@ function writeModules(modules, modulesDir, encoding) {
 
 function writeMagicFlagFile(outputDir) {
   /* global Buffer: true */
-  const buffer = Buffer(4);
-  buffer.writeUInt32LE(MAGIC_UNBUNDLE_NUMBER);
+  const buffer = new Buffer(4);
+  buffer.writeUInt32LE(MAGIC_UNBUNDLE_NUMBER, 0);
   return writeFile(path.join(outputDir, MAGIC_UNBUNDLE_FILENAME), buffer);
 }
 
diff --git a/local-cli/bundle/output/unbundle/as-indexed-file.js b/local-cli/bundle/output/unbundle/as-indexed-file.js
index 573a0df5a43..05978ec8f8a 100644
--- a/local-cli/bundle/output/unbundle/as-indexed-file.js
+++ b/local-cli/bundle/output/unbundle/as-indexed-file.js
@@ -5,6 +5,8 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
  */
 'use strict';
 
@@ -15,6 +17,10 @@ const fs = require('fs');
 const writeSourceMap = require('./write-sourcemap');
 
 const {joinModules} = require('./util');
+
+import type ModuleTransport from '../../../../packager/react-packager/src/lib/ModuleTransport';
+import type {Bundle, ModuleGroups, OutputOptions} from '../../types.flow';
+
 const SIZEOF_UINT32 = 4;
 
 /**
@@ -24,18 +30,22 @@ const SIZEOF_UINT32 = 4;
  * The module id for the startup code (prelude, polyfills etc.) is the
  * empty string.
  */
-function saveAsIndexedFile(bundle, options, log) {
+function saveAsIndexedFile(
+  bundle: Bundle,
+  options: OutputOptions,
+  log: (x: string) => void,
+): Promise<> {
   const {
     bundleOutput,
     bundleEncoding: encoding,
-    sourcemapOutput
+    sourcemapOutput,
   } = options;
 
   log('start');
   const {startupModules, lazyModules, groups} = bundle.getUnbundle();
   log('finish');
 
-  const moduleGroups = ModuleGroups(groups, lazyModules);
+  const moduleGroups = createModuleGroups(groups, lazyModules);
   const startupCode = joinModules(startupModules);
 
   log('Writing unbundle output to:', bundleOutput);
@@ -45,21 +55,26 @@ function saveAsIndexedFile(bundle, options, log) {
   ).then(() => log('Done writing unbundle output'));
 
   const sourceMap =
-    buildSourceMapWithMetaData({startupModules, lazyModules, moduleGroups});
+    buildSourceMapWithMetaData({
+      startupModules: startupModules.concat(),
+      lazyModules: lazyModules.concat(),
+      moduleGroups,
+    });
 
   return Promise.all([
     writeUnbundle,
-    writeSourceMap(sourcemapOutput, JSON.stringify(sourceMap), log),
+    sourcemapOutput && writeSourceMap(sourcemapOutput, JSON.stringify(sourceMap), log),
   ]);
 }
 
 /* global Buffer: true */
 
-const fileHeader = Buffer(4);
-fileHeader.writeUInt32LE(MAGIC_UNBUNDLE_FILE_HEADER);
-const nullByteBuffer = Buffer(1).fill(0);
+const fileHeader = new Buffer(4);
+fileHeader.writeUInt32LE(MAGIC_UNBUNDLE_FILE_HEADER, 0);
+//$FlowIssue #14640206
+const nullByteBuffer: Buffer = new Buffer(1).fill(0);
 
-function writeBuffers(stream, buffers) {
+function writeBuffers(stream, buffers: Array<Buffer>) {
   buffers.forEach(buffer => stream.write(buffer));
   return new Promise((resolve, reject) => {
     stream.on('error', reject);
@@ -69,7 +84,7 @@ function writeBuffers(stream, buffers) {
 }
 
 function nullTerminatedBuffer(contents, encoding) {
-  return Buffer.concat([Buffer(contents, encoding), nullByteBuffer]);
+  return Buffer.concat([new Buffer(contents, encoding), nullByteBuffer]);
 }
 
 function moduleToBuffer(id, code, encoding) {
@@ -98,7 +113,8 @@ function buildModuleTable(startupCode, buffers, moduleGroups) {
   const moduleIds = Array.from(moduleGroups.modulesById.keys());
   const maxId = moduleIds.reduce((max, id) => Math.max(max, id));
   const numEntries = maxId + 1;
-  const table = new Buffer(entryOffset(numEntries)).fill(0);
+  //$FlowIssue #14640206
+  const table: Buffer = new Buffer(entryOffset(numEntries)).fill(0);
 
   // num_entries
   table.writeUInt32LE(numEntries, 0);
@@ -109,9 +125,8 @@ function buildModuleTable(startupCode, buffers, moduleGroups) {
   // entries
   let codeOffset = startupCode.length;
   buffers.forEach(({id, buffer}) => {
-    const idsInGroup = moduleGroups.groups.has(id)
-      ? [id].concat(Array.from(moduleGroups.groups.get(id)))
-      : [id];
+    const group = moduleGroups.groups.get(id);
+    const idsInGroup = group ? [id].concat(Array.from(group)) : [id];
 
     idsInGroup.forEach(moduleId => {
       const offset = entryOffset(moduleId);
@@ -132,7 +147,7 @@ function groupCode(rootCode, moduleGroup, modulesById) {
   }
   const code = [rootCode];
   for (const id of moduleGroup) {
-    code.push(modulesById.get(id).code);
+    code.push((modulesById.get(id) || {}).code);
   }
 
   return code.join('\n');
@@ -170,7 +185,10 @@ function buildTableAndContents(startupCode, modules, moduleGroups, encoding) {
   ].concat(moduleBuffers.map(({buffer}) => buffer));
 }
 
-function ModuleGroups(groups, modules) {
+function createModuleGroups(
+  groups: Map<number, Set<number>>,
+  modules: Array<ModuleTransport>,
+): ModuleGroups {
   return {
     groups,
     modulesById: new Map(modules.map(m => [m.id, m])),
diff --git a/local-cli/bundle/output/unbundle/build-unbundle-sourcemap-with-metadata.js b/local-cli/bundle/output/unbundle/build-unbundle-sourcemap-with-metadata.js
index 0d69657c8a6..74c2348fb1b 100644
--- a/local-cli/bundle/output/unbundle/build-unbundle-sourcemap-with-metadata.js
+++ b/local-cli/bundle/output/unbundle/build-unbundle-sourcemap-with-metadata.js
@@ -5,14 +5,25 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
  */
 'use strict';
 
 const {combineSourceMaps, joinModules} = require('./util');
 
-module.exports = ({startupModules, lazyModules, moduleGroups}) => {
-  const startupModule = {
+import type {ModuleGroups, ModuleTransportLike} from '../../types.flow';
+
+type Params = {
+  lazyModules: Array<ModuleTransportLike>,
+  moduleGroups?: ModuleGroups,
+  startupModules: Array<ModuleTransportLike>,
+};
+
+module.exports = ({startupModules, lazyModules, moduleGroups}: Params) => {
+  const startupModule: ModuleTransportLike = {
     code: joinModules(startupModules),
+    id:  Number.MIN_SAFE_INTEGER,
     map: combineSourceMaps({modules: startupModules}),
   };
   return combineSourceMaps({
diff --git a/local-cli/bundle/output/unbundle/index.js b/local-cli/bundle/output/unbundle/index.js
index 8b9e9cda17d..aaa9c2de66c 100644
--- a/local-cli/bundle/output/unbundle/index.js
+++ b/local-cli/bundle/output/unbundle/index.js
@@ -5,13 +5,19 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
  */
 'use strict';
 
 const asAssets = require('./as-assets');
 const asIndexedFile = require('./as-indexed-file');
 
-function buildBundle(packagerClient, requestOptions) {
+import type Bundle from '../../../../packager/react-packager/src/Bundler/Bundle';
+import type Server from '../../../../packager/react-packager/src/Server';
+import type {OutputOptions, RequestOptions} from '../../types.flow';
+
+function buildBundle(packagerClient: Server, requestOptions: RequestOptions) {
   return packagerClient.buildBundle({
     ...requestOptions,
     unbundle: true,
@@ -19,7 +25,11 @@ function buildBundle(packagerClient, requestOptions) {
   });
 }
 
-function saveUnbundle(bundle, options, log) {
+function saveUnbundle(
+  bundle: Bundle,
+  options: OutputOptions,
+  log: (x: string) => void,
+): Promise<mixed> {
   // we fork here depending on the platform:
   // while android is pretty good at loading individual assets, ios has a large
   // overhead when reading hundreds pf assets from disk
diff --git a/local-cli/bundle/output/unbundle/magic-number.js b/local-cli/bundle/output/unbundle/magic-number.js
index 6e6a9d3759d..64293f8762a 100644
--- a/local-cli/bundle/output/unbundle/magic-number.js
+++ b/local-cli/bundle/output/unbundle/magic-number.js
@@ -5,6 +5,8 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
  */
 'use strict';
 
diff --git a/local-cli/bundle/output/unbundle/util.js b/local-cli/bundle/output/unbundle/util.js
index 16458bae10e..ab48c31a075 100644
--- a/local-cli/bundle/output/unbundle/util.js
+++ b/local-cli/bundle/output/unbundle/util.js
@@ -5,14 +5,19 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
  */
 'use strict';
 
-const newline = /\r\n?|\n|\u2028|\u2029/g;
-const countLines =
-  string => (string.match(newline) || []).length + 1; // fastest implementation
+import type {ModuleGroups, ModuleTransportLike, SourceMap} from '../../types.flow';
 
-function lineToLineSourceMap(source, filename) {
+const newline = /\r\n?|\n|\u2028|\u2029/g;
+// fastest implementation
+const countLines = (string: string) => (string.match(newline) || []).length + 1;
+
+
+function lineToLineSourceMap(source: string, filename: string = ''): SourceMap {
   // The first line mapping in our package is the base64vlq code for zeros (A).
   const firstLine = 'AAAA;';
 
@@ -21,34 +26,52 @@ function lineToLineSourceMap(source, filename) {
   const line = 'AACA;';
 
   return {
-    version: 3,
-    sources: [filename],
+    file: filename,
     mappings: firstLine + Array(countLines(source)).join(line),
+    sources: [filename],
+    names: [],
+    version: 3,
   };
 }
 
 const wrapperEnd = wrappedCode => wrappedCode.indexOf('{') + 1;
 
-const Section = (line, column, map) => ({map, offset: {line, column}});
+const Section =
+  (line: number, column: number, map: SourceMap) =>
+    ({map, offset: {line, column}});
 
-function combineSourceMaps({modules, withCustomOffsets, moduleGroups}) {
+type CombineSourceMapsOptions = {
+  moduleGroups?: ModuleGroups,
+  modules: Array<ModuleTransportLike>,
+  withCustomOffsets?: boolean,
+};
+
+function combineSourceMaps({
+  moduleGroups,
+  modules,
+  withCustomOffsets,
+}: CombineSourceMapsOptions): SourceMap {
   let offsets;
   const sections = [];
-  const sourceMap = {
-    version: 3,
+  const sourceMap: Object = {
+    file: '',
     sections,
+    version: 3,
   };
 
+
   if (withCustomOffsets) {
     offsets = sourceMap.x_facebook_offsets = [];
   }
 
   let line = 0;
-  modules.forEach(({code, id, map, name}) => {
+  modules.forEach(moduleTransport => {
+    const {code, id, name} = moduleTransport;
     let column = 0;
     let hasOffset = false;
     let group;
     let groupLines = 0;
+    let {map} = moduleTransport;
 
     if (withCustomOffsets) {
       if (moduleGroups && moduleGroups.modulesInGroups.has(id)) {
@@ -56,15 +79,19 @@ function combineSourceMaps({modules, withCustomOffsets, moduleGroups}) {
         return;
       }
 
-      if (moduleGroups && moduleGroups.groups.has(id)) {
-        group = moduleGroups.groups.get(id);
-        const otherModules = Array.from(group).map(
-          moduleId => moduleGroups.modulesById.get(moduleId));
+
+      group = moduleGroups && moduleGroups.groups.get(id);
+      if (group && moduleGroups) {
+        const {modulesById} = moduleGroups;
+        const otherModules: Array<ModuleTransportLike> =
+          Array.from(group || [])
+            .map(moduleId => modulesById.get(moduleId))
+            .filter(Boolean); // needed to appease flow
         otherModules.forEach(m => {
           groupLines += countLines(m.code);
         });
         map = combineSourceMaps({
-          modules: [{code, id, map, name}].concat(otherModules),
+          modules: [moduleTransport].concat(otherModules),
         });
       }
 
@@ -85,7 +112,9 @@ function combineSourceMaps({modules, withCustomOffsets, moduleGroups}) {
   return sourceMap;
 }
 
-const joinModules = modules => modules.map(m => m.code).join('\n');
+const joinModules =
+  (modules: Array<*>): string =>
+    modules.map(m => m.code).join('\n');
 
 module.exports = {
   countLines,
diff --git a/local-cli/bundle/output/unbundle/write-sourcemap.js b/local-cli/bundle/output/unbundle/write-sourcemap.js
index f10e8aa2dc2..35af1b77b67 100644
--- a/local-cli/bundle/output/unbundle/write-sourcemap.js
+++ b/local-cli/bundle/output/unbundle/write-sourcemap.js
@@ -5,12 +5,18 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
  */
 'use strict';
 
 const writeFile = require('../writeFile');
 
-function writeSourcemap(fileName, contents, log) {
+function writeSourcemap(
+  fileName: string,
+  contents: string,
+  log: (x: string) => void,
+): Promise<> {
   if (!fileName) {
     return Promise.resolve();
   }
diff --git a/local-cli/bundle/output/writeFile.js b/local-cli/bundle/output/writeFile.js
index 794b39020d1..445ccef2259 100644
--- a/local-cli/bundle/output/writeFile.js
+++ b/local-cli/bundle/output/writeFile.js
@@ -5,20 +5,16 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
  */
 'use strict';
 
+const denodeify = require('denodeify');
 const fs = require('fs');
 
-function writeFile(file, data, encoding) {
-  return new Promise((resolve, reject) => {
-    fs.writeFile(
-      file,
-      data,
-      encoding,
-      error => error ? reject(error) : resolve()
-    );
-  });
-}
+type WriteFn =
+  (file: string, data: string | Buffer, encoding?: ?string) => Promise<mixed>;
+const writeFile: WriteFn = denodeify(fs.writeFile);
 
 module.exports = writeFile;
diff --git a/local-cli/bundle/types.flow.js b/local-cli/bundle/types.flow.js
new file mode 100644
index 00000000000..d0768fef854
--- /dev/null
+++ b/local-cli/bundle/types.flow.js
@@ -0,0 +1,47 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
+ */
+'use strict';
+
+import type Bundle from '../../packager/react-packager/src/Bundler/Bundle';
+import type {Unbundle} from '../../packager/react-packager/src/Bundler/Bundle';
+import type ModuleTransport from '../../packager/react-packager/src/lib/ModuleTransport';
+import type {MixedSourceMap} from '../../packager/react-packager/src/lib/SourceMap';
+
+export type {Bundle, ModuleTransport, MixedSourceMap as SourceMap, Unbundle};
+
+export type ModuleGroups = {|
+  groups: Map<number, Set<number>>,
+  modulesById: Map<number, ModuleTransport>,
+  modulesInGroups: Set<number>,
+|};
+
+export type ModuleTransportLike = {
+  code: string,
+  id: number,
+  map?: ?MixedSourceMap,
+  +name?: string,
+};
+
+export type OutputOptions = {
+  bundleOutput: string,
+  bundleEncoding?: 'utf8' | 'utf16le' | 'ascii',
+  dev?: boolean,
+  platform: string,
+  sourcemapOutput?: string,
+};
+
+export type RequestOptions = {|
+  entryFile: string,
+  sourceMapUrl?: string,
+  dev?: boolean,
+  minify: boolean,
+  platform: string,
+|};
diff --git a/packager/react-packager/src/Bundler/Bundle.js b/packager/react-packager/src/Bundler/Bundle.js
index 37b6f5e677d..bf4066960f5 100644
--- a/packager/react-packager/src/Bundler/Bundle.js
+++ b/packager/react-packager/src/Bundler/Bundle.js
@@ -21,6 +21,12 @@ const crypto = require('crypto');
 import type {SourceMap, CombinedSourceMap, MixedSourceMap} from '../lib/SourceMap';
 import type {GetSourceOptions, FinalizeOptions} from './BundleBase';
 
+export type Unbundle = {
+  startupModules: Array<*>,
+  lazyModules: Array<*>,
+  groups: Map<number, Set<number>>,
+};
+
 const SOURCEMAPPING_URL = '\n\/\/# sourceMappingURL=';
 
 class Bundle extends BundleBase {
@@ -29,7 +35,7 @@ class Bundle extends BundleBase {
   _inlineSourceMap: string | void;
   _minify: boolean | void;
   _numRequireCalls: number;
-  _ramBundle: mixed | void;
+  _ramBundle: Unbundle | null;
   _ramGroups: Array<string> | void;
   _shouldCombineSourceMaps: boolean;
   _sourceMap: boolean;
@@ -144,7 +150,7 @@ class Bundle extends BundleBase {
     return source;
   }
 
-  getUnbundle() {
+  getUnbundle(): Unbundle {
     this.assertFinalized();
     if (!this._ramBundle) {
       const modules = this.getModules().slice();
@@ -419,7 +425,7 @@ function createGroups(ramGroups: Array<string>, lazyModules) {
   });
 
   // build a map of group root IDs to an array of module IDs in the group
-  const result = new Map(
+  const result: Map<number, Set<number>> = new Map(
     ramGroups
       .map(modulePath => {
         const root = byPath.get(modulePath);
diff --git a/packager/react-packager/src/Bundler/BundleBase.js b/packager/react-packager/src/Bundler/BundleBase.js
index 35c8a56dbbd..a39a14f6670 100644
--- a/packager/react-packager/src/Bundler/BundleBase.js
+++ b/packager/react-packager/src/Bundler/BundleBase.js
@@ -19,7 +19,7 @@ export type FinalizeOptions = {
 };
 
 export type GetSourceOptions = {
-  inlineSourceMap: boolean,
+  inlineSourceMap?: boolean,
   dev: boolean,
 };
 
diff --git a/packager/react-packager/src/lib/ModuleTransport.js b/packager/react-packager/src/lib/ModuleTransport.js
index 51a7780299a..1222ac9399c 100644
--- a/packager/react-packager/src/lib/ModuleTransport.js
+++ b/packager/react-packager/src/lib/ModuleTransport.js
@@ -11,7 +11,7 @@
 
 'use strict';
 
-import type {SourceMap} from './SourceMap';
+import type {MixedSourceMap} from './SourceMap';
 
 type Metadata = {
   dependencyPairs?: Array<[mixed, {path: string}]>,
@@ -21,25 +21,25 @@ type Metadata = {
 class ModuleTransport {
 
   name: string;
-  id: string | number;
+  id: number;
   code: string;
   sourceCode: string;
   sourcePath: string;
   virtual: ?boolean;
   meta: ?Metadata;
   polyfill: ?boolean;
-  map: ?SourceMap;
+  map: ?MixedSourceMap;
 
   constructor(data: {
     name: string,
-    id: string | number,
+    id: number,
     code: string,
     sourceCode: string,
     sourcePath: string,
     virtual?: ?boolean,
     meta?: ?Metadata,
     polyfill?: ?boolean,
-    map?: ?SourceMap,
+    map?: ?MixedSourceMap,
   }) {
     this.name = data.name;
 
diff --git a/packager/react-packager/src/lib/SourceMap.js b/packager/react-packager/src/lib/SourceMap.js
index a10ab619aff..5ffa42f0bb2 100644
--- a/packager/react-packager/src/lib/SourceMap.js
+++ b/packager/react-packager/src/lib/SourceMap.js
@@ -20,8 +20,14 @@ export type CombinedSourceMap = {
   file: string,
   sections: Array<{
     offset: {line: number, column: number},
-    map: SourceMap,
+    map: MixedSourceMap,
   }>,
 };
 
-export type MixedSourceMap = SourceMap | CombinedSourceMap;
+type FBExtensions = {x_facebook_offsets?: Array<number>};
+
+export type MixedSourceMap
+  = SourceMap
+  | CombinedSourceMap
+  | (SourceMap & FBExtensions)
+  | (CombinedSourceMap & FBExtensions);