diff --git a/packager/src/Bundler/index.js b/packager/src/Bundler/index.js
index d5da5d7cd14..60edab60380 100644
--- a/packager/src/Bundler/index.js
+++ b/packager/src/Bundler/index.js
@@ -642,7 +642,7 @@ class Bundler {
     entryFilePath: string,
     options: BundlingOptions,
     getModuleId: () => number,
-    dependencyPairs: Array<[mixed, {path: string}]>,
+    dependencyPairs: Array<[string, Module]>,
     assetPlugins: Array<string>,
   }): Promise<ModuleTransport> {
     let moduleTransport;
diff --git a/packager/src/Server/index.js b/packager/src/Server/index.js
index f9dcf6e626a..fae8323b7c2 100644
--- a/packager/src/Server/index.js
+++ b/packager/src/Server/index.js
@@ -552,6 +552,7 @@ class Server {
               changedModules.forEach(m => {
                 response.setResolvedDependencyPairs(
                   m,
+                  /* $FlowFixMe: should be enforced not to be null. */
                   dependencyPairs.get(m.path),
                   {ignoreFinalized: true},
                 );
diff --git a/packager/src/lib/ModuleTransport.js b/packager/src/lib/ModuleTransport.js
index 1c4e48bdd86..4ba202673ea 100644
--- a/packager/src/lib/ModuleTransport.js
+++ b/packager/src/lib/ModuleTransport.js
@@ -12,13 +12,14 @@
 'use strict';
 
 import type {RawMapping} from '../Bundler/source-map';
+import type Module from '../node-haste/Module';
 import type {SourceMap} from './SourceMap';
 
 type SourceMapOrMappings = SourceMap | Array<RawMapping>;
 
 type Metadata = {
   dependencies?: ?Array<string>,
-  dependencyPairs?: Array<[mixed, {path: string}]>,
+  dependencyPairs?: Array<[string, Module]>,
   preloaded: ?boolean,
 };
 
diff --git a/packager/src/node-haste/DependencyGraph/ResolutionRequest.js b/packager/src/node-haste/DependencyGraph/ResolutionRequest.js
index 4a973778db1..da674da3ba8 100644
--- a/packager/src/node-haste/DependencyGraph/ResolutionRequest.js
+++ b/packager/src/node-haste/DependencyGraph/ResolutionRequest.js
@@ -214,9 +214,10 @@ class ResolutionRequest<TModule: Moduleish, TPackage: Packageish> {
           );
       });
 
-    const collectedDependencies = new MapWithDefaults(module =>
-      collect(module),
-    );
+    const collectedDependencies: MapWithDefaults<
+      TModule,
+      Promise<Array<TModule>>,
+    > = new MapWithDefaults(module => collect(module));
     const crawlDependencies = (mod, [depNames, dependencies]) => {
       const filteredPairs = [];
 
diff --git a/packager/src/node-haste/DependencyGraph/ResolutionResponse.js b/packager/src/node-haste/DependencyGraph/ResolutionResponse.js
index 2b1bbb134a9..1b3eca62579 100644
--- a/packager/src/node-haste/DependencyGraph/ResolutionResponse.js
+++ b/packager/src/node-haste/DependencyGraph/ResolutionResponse.js
@@ -11,8 +11,6 @@
 
 'use strict';
 
-import type Module from '../Module';
-
 const NO_OPTIONS = {};
 
 class ResolutionResponse<TModule: {hash(): string}, TOptions> {
@@ -26,7 +24,7 @@ class ResolutionResponse<TModule: {hash(): string}, TOptions> {
   // This is monkey-patched from Resolver.
   getModuleId: ?() => number;
 
-  _mappings: {};
+  _mappings: {[hash: string]: Array<[string, TModule]>};
   _finalized: boolean;
   _mainModule: ?TModule;
 
@@ -104,8 +102,8 @@ class ResolutionResponse<TModule: {hash(): string}, TOptions> {
   }
 
   setResolvedDependencyPairs(
-    module: Module,
-    pairs: mixed,
+    module: TModule,
+    pairs: Array<[string, TModule]>,
     options: {ignoreFinalized?: boolean} = NO_OPTIONS,
   ) {
     if (!options.ignoreFinalized) {
@@ -121,7 +119,7 @@ class ResolutionResponse<TModule: {hash(): string}, TOptions> {
     this.mocks = mocks;
   }
 
-  getResolvedDependencyPairs(module: TModule) {
+  getResolvedDependencyPairs(module: TModule): $ReadOnlyArray<[string, TModule]> {
     this._assertFinalized();
     return this._mappings[module.hash()];
   }
diff --git a/packager/src/node-haste/lib/MapWithDefaults.js b/packager/src/node-haste/lib/MapWithDefaults.js
index a264f0c2518..ce79e045051 100644
--- a/packager/src/node-haste/lib/MapWithDefaults.js
+++ b/packager/src/node-haste/lib/MapWithDefaults.js
@@ -1,30 +1,35 @@
- /**
+/**
  * Copyright (c) 2016-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
+ * @format
  */
+
 'use strict';
 
-module.exports = function MapWithDefaults(factory, iterable) {
-  // This can't be `MapWithDefaults extends Map`, b/c the way babel transforms
-  // super calls in constructors: Map.call(this, iterable) throws for native
-  // Map objects in node 4+.
-  // TODO(davidaurelio) switch to a transform that does not transform classes
-  // and super calls, and change this into a class
+class MapWithDefaults<TK, TV> extends Map<TK, TV> {
+  _factory: TK => TV;
 
-  const map = iterable ? new Map(iterable) : new Map();
-  const {get} = map;
-  map.get = key => {
-    if (map.has(key)) {
-      return get.call(map, key);
+  constructor(factory: TK => TV, iterable?: Iterable<[TK, TV]>) {
+    super(iterable);
+    this._factory = factory;
+  }
+
+  get(key: TK): TV {
+    if (this.has(key)) {
+      /* $FlowFixMe: can never be `undefined` since we tested with `has`
+       * (except if `TV` includes `void` as subtype, ex. is nullable) */
+      return Map.prototype.get.call(this, key);
     }
-
-    const value = factory(key);
-    map.set(key, value);
+    const value = this._factory(key);
+    this.set(key, value);
     return value;
-  };
-  return map;
-};
+  }
+}
+
+module.exports = MapWithDefaults;
diff --git a/packager/src/node-haste/lib/__tests__/MapWithDefaults-test.js b/packager/src/node-haste/lib/__tests__/MapWithDefaults-test.js
new file mode 100644
index 00000000000..efb122b3d7b
--- /dev/null
+++ b/packager/src/node-haste/lib/__tests__/MapWithDefaults-test.js
@@ -0,0 +1,24 @@
+/**
+ * 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.
+ *
+ * @format
+ */
+
+'use strict';
+
+jest.disableAutomock();
+
+const MapWithDefaults = require('../MapWithDefaults');
+
+describe('MapWithDefaults', function() {
+  it('works', () => {
+    const map = new MapWithDefaults(() => ['bar']);
+    map.get('foo').push('baz');
+    expect(map.get('foo')).toEqual(['bar', 'baz']);
+  });
+});
diff --git a/packager/src/node-haste/lib/getInverseDependencies.js b/packager/src/node-haste/lib/getInverseDependencies.js
index 6ff90334f87..663208ee760 100644
--- a/packager/src/node-haste/lib/getInverseDependencies.js
+++ b/packager/src/node-haste/lib/getInverseDependencies.js
@@ -17,7 +17,7 @@ import type ResolutionResponse from '../DependencyGraph/ResolutionResponse';
 function resolveModuleRequires<TModule: {hash(): string}, TOptions>(
   resolutionResponse: ResolutionResponse<TModule, TOptions>,
   module: TModule,
-) {
+): Array<TModule> {
   const pairs = resolutionResponse.getResolvedDependencyPairs(module);
   return pairs ? pairs.map(([, dependencyModule]) => dependencyModule) : [];
 }