Add AsyncMemoizer.future.

test/async_memoizer_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 
 import 'package:async/async.dart';
 import 'package:test/test.dart';
 
 main() {
   var cache;
   setUp(() => cache = new AsyncMemoizer());
 
   test("runs the function only the first time runOnce() is called", () async {
     var count = 0;
     expect(await cache.runOnce(() => count++), equals(0));
     expect(count, equals(1));
 
     expect(await cache.runOnce(() => count++), equals(0));
     expect(count, equals(1));
   });
 
   test("forwards the return value from the function", () async {
+    expect(cache.future, completion(equals("value")));
     expect(cache.runOnce(() => "value"), completion(equals("value")));
     expect(cache.runOnce(() {}), completion(equals("value")));
   });
 
   test("forwards the return value from an async function", () async {
+    expect(cache.future, completion(equals("value")));
     expect(cache.runOnce(() async => "value"), completion(equals("value")));
     expect(cache.runOnce(() {}), completion(equals("value")));
   });
 
   test("forwards the error from an async function", () async {
+    expect(cache.future, throwsA("error"));
     expect(cache.runOnce(() async => throw "error"), throwsA("error"));
     expect(cache.runOnce(() {}), throwsA("error"));
   });
 }