Dart: Better handle leak checks. close() is async.

services/dart/dart_apptests/pingpong_apptests.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+library pingpong_apptests;
+
 import 'dart:async';
 import 'mojo:application';
 import 'mojo:bindings';
 import 'mojo:core';
 
 import 'package:apptest/apptest.dart';
 import 'package:services/dart/test/pingpong_service.mojom.dart';
 
 class _TestingPingPongClient extends PingPongClient {
   final PingPongClientStub stub;
@@ skipping 89 matching lines @@
       targetServiceProxy.ptr.ping(100);
       pongValue = await pingPongClient.waitForPong();
       expect(pongValue, equals(101));
 
       pingPongClient.stub.close();
       targetServiceProxy.close();
       pingPongServiceProxy.close();
     });
   });
 }