diff --git a/packages/metro-bundler/src/node-haste/lib/AsyncTaskGroup.js b/packages/metro-bundler/src/node-haste/lib/AsyncTaskGroup.js
index 7f99b25e..81235595 100644
--- a/packages/metro-bundler/src/node-haste/lib/AsyncTaskGroup.js
+++ b/packages/metro-bundler/src/node-haste/lib/AsyncTaskGroup.js
@@ -5,23 +5,31 @@
  * 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';
 
-module.exports = class AsyncTaskGroup {
+module.exports = class AsyncTaskGroup<TTaskHandle> {
+  _runningTasks: Set<TTaskHandle>;
+  _resolve: ?() => void;
+  done: Promise<void>;
+
   constructor() {
     this._runningTasks = new Set();
     this._resolve = null;
     this.done = new Promise(resolve => this._resolve = resolve);
   }
 
-  start(taskHandle) {
+  start(taskHandle: TTaskHandle) {
     this._runningTasks.add(taskHandle);
   }
 
-  end(taskHandle) {
+  end(taskHandle: TTaskHandle) {
     const runningTasks = this._runningTasks;
     if (runningTasks.delete(taskHandle) && runningTasks.size === 0) {
+      /* $FlowFixMe: could be null */
       this._resolve();
     }
   }