diff --git a/react-packager/src/Bundler/Bundle.js b/react-packager/src/Bundler/Bundle.js
index 37b6f5e6..bf406696 100644
--- a/react-packager/src/Bundler/Bundle.js
+++ b/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/react-packager/src/Bundler/BundleBase.js b/react-packager/src/Bundler/BundleBase.js
index 35c8a56d..a39a14f6 100644
--- a/react-packager/src/Bundler/BundleBase.js
+++ b/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/react-packager/src/lib/ModuleTransport.js b/react-packager/src/lib/ModuleTransport.js
index 51a77802..1222ac93 100644
--- a/react-packager/src/lib/ModuleTransport.js
+++ b/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/react-packager/src/lib/SourceMap.js b/react-packager/src/lib/SourceMap.js
index a10ab619..5ffa42f0 100644
--- a/react-packager/src/lib/SourceMap.js
+++ b/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);