diff --git a/packager/react-packager/src/Bundler/Bundle.js b/packager/react-packager/src/Bundler/Bundle.js
index 7bce385e2..37b6f5e67 100644
--- a/packager/react-packager/src/Bundler/Bundle.js
+++ b/packager/react-packager/src/Bundler/Bundle.js
@@ -18,6 +18,7 @@ const _ = require('lodash');
 const base64VLQ = require('./base64-vlq');
 const crypto = require('crypto');
 
+import type {SourceMap, CombinedSourceMap, MixedSourceMap} from '../lib/SourceMap';
 import type {GetSourceOptions, FinalizeOptions} from './BundleBase';
 
 const SOURCEMAPPING_URL = '\n\/\/# sourceMappingURL=';
@@ -53,14 +54,18 @@ class Bundle extends BundleBase {
   }
 
   addModule(
+    /**
+     * $FlowFixMe: this code is inherently incorrect, because it modifies the
+     * signature of the base class function "addModule". That means callsites
+     * using an instance typed as the base class would be broken. This must be
+     * refactored.
+     */
     resolver: {wrapModule: (options: any) => Promise<{code: any, map: any}>},
     resolutionResponse: mixed,
     module: mixed,
+    /* $FlowFixMe: erroneous change of signature. */
     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. */
+    /* $FlowFixMe: erroneous change of signature. */
   ): Promise<void> {
     const index = super.addModule(moduleTransport);
     return resolver.wrapModule({
@@ -170,7 +175,7 @@ class Bundle extends BundleBase {
    * that makes use of of the `sections` field to combine sourcemaps by adding
    * an offset. This is supported only by Chrome for now.
    */
-  _getCombinedSourceMaps(options) {
+  _getCombinedSourceMaps(options): CombinedSourceMap {
     const result = {
       version: 3,
       file: this._getSourceMapFile(),
@@ -186,6 +191,7 @@ class Bundle extends BundleBase {
       }
 
       if (options.excludeSource) {
+        /* $FlowFixMe: assume the map is not empty if we got here. */
         if (map.sourcesContent && map.sourcesContent.length) {
           map = Object.assign({}, map, {sourcesContent: []});
         }
@@ -193,6 +199,7 @@ class Bundle extends BundleBase {
 
       result.sections.push({
         offset: { line: line, column: 0 },
+        /* $FlowFixMe: assume the map is not empty if we got here. */
         map: map,
       });
       line += module.code.split('\n').length;
@@ -201,7 +208,7 @@ class Bundle extends BundleBase {
     return result;
   }
 
-  getSourceMap(options: {excludeSource?: boolean}) {
+  getSourceMap(options: {excludeSource?: boolean}): MixedSourceMap {
     super.assertFinalized();
 
     if (this._shouldCombineSourceMaps) {
@@ -336,11 +343,12 @@ class Bundle extends BundleBase {
 
     BundleBase.fromJSON(bundle, json);
 
+    /* $FlowFixMe: this modifies BundleBase#fromJSON() signature. */
     return bundle;
   }
 }
 
-function generateSourceMapForVirtualModule(module) {
+function generateSourceMapForVirtualModule(module): SourceMap {
   // All lines map 1-to-1
   let mappings = 'AAAA;';
 
@@ -379,6 +387,7 @@ function * filter(iterator, predicate) {
 
 function * subtree(moduleTransport: ModuleTransport, moduleTransportsByPath, seen = new Set()) {
   seen.add(moduleTransport.id);
+  /* $FlowFixMe: there may not be a `meta` object */
   for (const [, {path}] of moduleTransport.meta.dependencyPairs || []) {
     const dependency = moduleTransportsByPath.get(path);
     if (dependency && !seen.has(dependency.id)) {
@@ -420,6 +429,7 @@ function createGroups(ramGroups: Array<string>, lazyModules) {
         return [
           root.id,
           // `subtree` yields the IDs of all transitive dependencies of a module
+          /* $FlowFixMe: assumes the module is always in the Map */
           new Set(subtree(byPath.get(root.sourcePath), byPath)),
         ];
       })
diff --git a/packager/react-packager/src/Bundler/index.js b/packager/react-packager/src/Bundler/index.js
index 6e4b37954..4165edcd9 100644
--- a/packager/react-packager/src/Bundler/index.js
+++ b/packager/react-packager/src/Bundler/index.js
@@ -329,7 +329,7 @@ class Bundler {
     };
     const finalizeBundle = ({bundle: finalBundle, transformedModules, response, modulesByName}: {
       bundle: Bundle,
-      transformedModules: Array<{module: Module, transformed: {}}>,
+      transformedModules: Array<{module: Module, transformed: ModuleTransport}>,
       response: ResolutionResponse,
       modulesByName: {[name: string]: Module},
     }) =>
diff --git a/packager/react-packager/src/lib/ModuleTransport.js b/packager/react-packager/src/lib/ModuleTransport.js
index e5718e2dc..51a778029 100644
--- a/packager/react-packager/src/lib/ModuleTransport.js
+++ b/packager/react-packager/src/lib/ModuleTransport.js
@@ -5,30 +5,64 @@
  * 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';
 
-function ModuleTransport(data) {
-  this.name = data.name;
+import type {SourceMap} from './SourceMap';
 
-  assertExists(data, 'id');
-  this.id = data.id;
+type Metadata = {
+  dependencyPairs?: Array<[mixed, {path: string}]>,
+  preloaded?: boolean,
+};
 
-  assertExists(data, 'code');
-  this.code = data.code;
+class ModuleTransport {
 
-  assertExists(data, 'sourceCode');
-  this.sourceCode = data.sourceCode;
+  name: string;
+  id: string | number;
+  code: string;
+  sourceCode: string;
+  sourcePath: string;
+  virtual: ?boolean;
+  meta: ?Metadata;
+  polyfill: ?boolean;
+  map: ?SourceMap;
 
-  assertExists(data, 'sourcePath');
-  this.sourcePath = data.sourcePath;
+  constructor(data: {
+    name: string,
+    id: string | number,
+    code: string,
+    sourceCode: string,
+    sourcePath: string,
+    virtual?: ?boolean,
+    meta?: ?Metadata,
+    polyfill?: ?boolean,
+    map?: ?SourceMap,
+  }) {
+    this.name = data.name;
 
-  this.virtual = data.virtual;
-  this.meta = data.meta;
-  this.polyfill = data.polyfill;
-  this.map = data.map;
+    assertExists(data, 'id');
+    this.id = data.id;
+
+    assertExists(data, 'code');
+    this.code = data.code;
+
+    assertExists(data, 'sourceCode');
+    this.sourceCode = data.sourceCode;
+
+    assertExists(data, 'sourcePath');
+    this.sourcePath = data.sourcePath;
+
+    this.virtual = data.virtual;
+    this.meta = data.meta;
+    this.polyfill = data.polyfill;
+    this.map = data.map;
+
+    Object.freeze(this);
+  }
 
-  Object.freeze(this);
 }
 
 module.exports = ModuleTransport;
diff --git a/packager/react-packager/src/lib/SourceMap.js b/packager/react-packager/src/lib/SourceMap.js
new file mode 100644
index 000000000..2f427f87c
--- /dev/null
+++ b/packager/react-packager/src/lib/SourceMap.js
@@ -0,0 +1,33 @@
+/**
+ * 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';
+
+type SourceMapBase = {
+  version: number,
+  file: string,
+};
+
+export type SourceMap = SourceMapBase & {
+  sources: Array<string>,
+  names: Array<string>,
+  mappings: string,
+  sourcesContent: Array<string>,
+};
+
+export type CombinedSourceMap = SourceMapBase & {
+  sections: Array<{
+    offset: {line: number, column: number},
+    map: SourceMap,
+  }>,
+};
+
+export type MixedSourceMap = SourceMap | CombinedSourceMap;