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

mojo/public/dart/src/proxy.dart

 // Copyright 2014 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.
 
 part of bindings;
 
 abstract class Proxy extends core.MojoEventStreamListener {
   Map<int, Completer> _completerMap;
   int _nextId = 0;
 
@@ skipping 67 matching lines @@
   Map<int, Completer> get completerMap => _completerMap;
 
   String toString() {
     var superString = super.toString();
     return "Proxy(${superString})";
   }
 }
 
 // Generated Proxy classes implement this interface.
 abstract class ProxyBase {
-  final Proxy impl;
-  final String name;
+  final Proxy impl = null;
+  final String name = null;
 }