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

services/dart/test/pingpong/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 100 matching lines @@
   void acceptConnection(String requestorUrl, String resolvedUrl,
       ApplicationConnection connection) {
     connection.provideService(PingPongServiceName,
         (endpoint) => new PingPongServiceImpl(this, endpoint));
   }
 }
 
 main(List args) {
   MojoHandle appHandle = new MojoHandle(args[0]);
   String url = args[1];
-  new PingPongApplication.fromHandle(appHandle);
+  new PingPongApplication.fromHandle(appHandle)
+    ..onError = (() {
+      assert(MojoHandle.reportLeakedHandles());
+    });
 }