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

services/dart/test/pingpong_target/main.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.
 
 import 'dart:async';
 import 'dart:mojo.application';
 import 'dart:mojo.bindings';
 import 'dart:mojo.core';
 
 import 'package:services/dart/test/pingpong_service.mojom.dart';
@@ skipping 34 matching lines @@
 
 class PingPongApplication extends Application {
   PingPongApplication.fromHandle(MojoHandle handle) : super.fromHandle(handle);
 
   @override
   void acceptConnection(String requestorUrl, String resolvedUrl,
       ApplicationConnection connection) {
     connection.provideService(PingPongServiceName,
         (endpoint) => new PingPongServiceImpl(this, endpoint));
     // Close the application when the first connection goes down.
-    connection.onError = close;
+    connection.onError = closeApplication;
+  }
+
+  Future closeApplication() async {
+    await close();
+    assert(MojoHandle.reportLeakedHandles());
   }
 }
 
 main(List args) {
   MojoHandle appHandle = new MojoHandle(args[0]);
   String url = args[1];
   new PingPongApplication.fromHandle(appHandle);
 }