diff --git a/react-packager/src/Bundler/Bundle.js b/react-packager/src/Bundler/Bundle.js
index b29af01a..7bce385e 100644
--- a/react-packager/src/Bundler/Bundle.js
+++ b/react-packager/src/Bundler/Bundle.js
@@ -5,19 +5,41 @@
  * 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 BundleBase = require('./BundleBase');
+const ModuleTransport = require('../lib/ModuleTransport');
+
 const _ = require('lodash');
 const base64VLQ = require('./base64-vlq');
-const BundleBase = require('./BundleBase');
-const ModuleTransport = require('../lib/ModuleTransport');
 const crypto = require('crypto');
 
+import type {GetSourceOptions, FinalizeOptions} from './BundleBase';
+
 const SOURCEMAPPING_URL = '\n\/\/# sourceMappingURL=';
 
 class Bundle extends BundleBase {
-  constructor({sourceMapUrl, dev, minify, ramGroups} = {}) {
+
+  _dev: boolean | void;
+  _inlineSourceMap: string | void;
+  _minify: boolean | void;
+  _numRequireCalls: number;
+  _ramBundle: mixed | void;
+  _ramGroups: Array<string> | void;
+  _shouldCombineSourceMaps: boolean;
+  _sourceMap: boolean;
+  _sourceMapUrl: string | void;
+
+  constructor({sourceMapUrl, dev, minify, ramGroups}: {
+    sourceMapUrl?: string,
+    dev?: boolean,
+    minify?: boolean,
+    ramGroups?: Array<string>,
+  } = {}) {
     super();
     this._sourceMap = false;
     this._sourceMapUrl = sourceMapUrl;
@@ -30,7 +52,16 @@ class Bundle extends BundleBase {
     this._ramBundle = null; // cached RAM Bundle
   }
 
-  addModule(resolver, resolutionResponse, module, moduleTransport) {
+  addModule(
+    resolver: {wrapModule: (options: any) => Promise<{code: any, map: any}>},
+    resolutionResponse: mixed,
+    module: mixed,
+    moduleTransport: ModuleTransport,
+    /* $FlowFixMe: this code is inherently incorrect, because it modifies the
+     * signature of the base class function "addModule", originally returning
+     * a number. That means callsites using an instance typed as the base class
+     * would be broken. This must be refactored. */
+  ): Promise<void> {
     const index = super.addModule(moduleTransport);
     return resolver.wrapModule({
       resolutionResponse,
@@ -53,17 +84,21 @@ class Bundle extends BundleBase {
     });
   }
 
-  finalize(options) {
+  finalize(options: FinalizeOptions) {
     options = options || {};
     if (options.runMainModule) {
+      /* $FlowFixMe: this is unsound, as nothing enforces runBeforeMainModule
+       * to be available if `runMainModule` is true. Refactor. */
       options.runBeforeMainModule.forEach(this._addRequireCall, this);
+      /* $FlowFixMe: this is unsound, as nothing enforces the module ID to have
+       * been set beforehand. */
       this._addRequireCall(super.getMainModuleId());
     }
 
     super.finalize(options);
   }
 
-  _addRequireCall(moduleId) {
+  _addRequireCall(moduleId: string) {
     const code = `;require(${JSON.stringify(moduleId)});`;
     const name = 'require-' + moduleId;
     super.addModule(new ModuleTransport({
@@ -88,12 +123,12 @@ class Bundle extends BundleBase {
     return this._inlineSourceMap;
   }
 
-  getSource(options) {
+  getSource(options: GetSourceOptions) {
     super.assertFinalized();
 
     options = options || {};
 
-    let source = super.getSource();
+    let source = super.getSource(options);
 
     if (options.inlineSourceMap) {
       source += SOURCEMAPPING_URL + this._getInlineSourceMap(options.dev);
@@ -104,7 +139,7 @@ class Bundle extends BundleBase {
     return source;
   }
 
-  getUnbundle(type) {
+  getUnbundle() {
     this.assertFinalized();
     if (!this._ramBundle) {
       const modules = this.getModules().slice();
@@ -122,7 +157,7 @@ class Bundle extends BundleBase {
             groups = createGroups(ramGroups || [], lazyModules);
           }
           return groups;
-        }
+        },
       };
     }
 
@@ -166,7 +201,7 @@ class Bundle extends BundleBase {
     return result;
   }
 
-  getSourceMap(options) {
+  getSourceMap(options: {excludeSource?: boolean}) {
     super.assertFinalized();
 
     if (this._shouldCombineSourceMaps) {
@@ -182,12 +217,14 @@ class Bundle extends BundleBase {
       names: [],
       mappings: mappings,
       sourcesContent: options.excludeSource
-        ? [] : modules.map(module => module.sourceCode)
+        ? [] : modules.map(module => module.sourceCode),
     };
     return map;
   }
 
   getEtag() {
+    /* $FlowFixMe: we must pass options, or rename the
+     * base `getSource` function, as it does not actually need options. */
     var eTag = crypto.createHash('md5').update(this.getSource()).digest('hex');
     return eTag;
   }
@@ -254,6 +291,7 @@ class Bundle extends BundleBase {
 
   getDebugInfo() {
     return [
+      /* $FlowFixMe: this is unsound as the module ID could be unset. */
       '<div><h3>Main Module:</h3> ' + super.getMainModuleId() + '</div>',
       '<style>',
       'pre.collapsed {',
@@ -274,7 +312,7 @@ class Bundle extends BundleBase {
     ].join('\n');
   }
 
-  setRamGroups(ramGroups) {
+  setRamGroups(ramGroups: Array<string>) {
     this._ramGroups = ramGroups;
   }
 
@@ -339,7 +377,7 @@ function * filter(iterator, predicate) {
   }
 }
 
-function * subtree(moduleTransport, moduleTransportsByPath, seen = new Set()) {
+function * subtree(moduleTransport: ModuleTransport, moduleTransportsByPath, seen = new Set()) {
   seen.add(moduleTransport.id);
   for (const [, {path}] of moduleTransport.meta.dependencyPairs || []) {
     const dependency = moduleTransportsByPath.get(path);
@@ -361,7 +399,7 @@ class ArrayMap extends Map {
   }
 }
 
-function createGroups(ramGroups, lazyModules) {
+function createGroups(ramGroups: Array<string>, lazyModules) {
   // build two maps that allow to lookup module data
   // by path or (numeric) module id;
   const byPath = new Map();
@@ -400,12 +438,14 @@ function createGroups(ramGroups, lazyModules) {
     const doubles = filter(all, ([, parents]) => parents.length > 1);
     for (const [moduleId, parents] of doubles) {
       // remove them from their groups
+      /* $FlowFixMe: this assumes the element exists. */
       parents.forEach(p => result.get(p).delete(moduleId));
 
       // print a warning for each removed module
       const parentNames = parents.map(byId.get, byId);
       const lastName = parentNames.pop();
       console.warn(
+        /* $FlowFixMe: this assumes the element exists. */
         `Module ${byId.get(moduleId)} belongs to groups ${
           parentNames.join(', ')}, and ${lastName
           }. Removing it from all groups.`
diff --git a/react-packager/src/Bundler/BundleBase.js b/react-packager/src/Bundler/BundleBase.js
index bf4c4f72..5f48b537 100644
--- a/react-packager/src/Bundler/BundleBase.js
+++ b/react-packager/src/Bundler/BundleBase.js
@@ -12,11 +12,22 @@
 
 const ModuleTransport = require('../lib/ModuleTransport');
 
+export type FinalizeOptions = {
+  allowUpdates?: boolean,
+  runBeforeMainModule?: Array<mixed>,
+  runMainModule?: boolean,
+};
+
+export type GetSourceOptions = {
+  inlineSourceMap: boolean,
+  dev: boolean,
+};
+
 class BundleBase {
 
   _assets: Array<mixed>;
   _finalized: boolean;
-  _mainModuleId: mixed | void;
+  _mainModuleId: string | void;
   _modules: Array<ModuleTransport>;
   _source: ?string;
 
@@ -35,7 +46,7 @@ class BundleBase {
     return this._mainModuleId;
   }
 
-  setMainModuleId(moduleId: mixed) {
+  setMainModuleId(moduleId: string) {
     this._mainModuleId = moduleId;
   }
 
@@ -67,7 +78,7 @@ class BundleBase {
     this._assets.push(asset);
   }
 
-  finalize(options: {allowUpdates?: boolean}) {
+  finalize(options: FinalizeOptions) {
     if (!options.allowUpdates) {
       Object.freeze(this._modules);
       Object.freeze(this._assets);
@@ -76,7 +87,7 @@ class BundleBase {
     this._finalized = true;
   }
 
-  getSource() {
+  getSource(options: GetSourceOptions) {
     this.assertFinalized();
 
     if (this._source) {
@@ -97,9 +108,9 @@ class BundleBase {
     }
   }
 
-  setRamGroups() {}
+  setRamGroups(ramGroups: Array<string>) {}
 
-  toJSON() {
+  toJSON(): mixed {
     return {
       modules: this._modules,
       assets: this._assets,