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

examples/dart/hello_world/world/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.
 
 // This app is run by examples/dart/hello_world/hello.
 
+import 'dart:async';
 import 'mojo:application';
 import 'mojo:bindings';
 import 'mojo:core';
 
 class World extends Application {
   World.fromHandle(MojoHandle handle) : super.fromHandle(handle);
 
   void initialize(List<String> args, String url) {
     print("$url World");
-    close();
+    closeApplication();
+  }
+
+  Future closeApplication() async {
+    await close();
+    assert(MojoHandle.reportLeakedHandles());
   }
 }
 
 main(List args) {
   MojoHandle appHandle = new MojoHandle(args[0]);
   new World.fromHandle(appHandle);
 }