Fix Microtask.dart

tools/dom/src/Microtask.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. 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 html;
 
 typedef void _MicrotaskCallback();
 
 /**
  * This class attempts to invoke a callback as soon as the current event stack
@@ skipping 43 matching lines @@
     }
     _nextMicrotaskFrameScheduled = false;
     this._callback();
   }
 }
 
 /**
  * Scheduler which uses window.postMessage to schedule events.
  */
 class _PostMessageScheduler extends _MicrotaskScheduler {
-  const _MICROTASK_MESSAGE = "DART-MICROTASK";
+  final _MICROTASK_MESSAGE = "DART-MICROTASK";
 
   _PostMessageScheduler(_MicrotaskCallback callback): super(callback) {
       // Messages from other windows do not cause a security risk as
       // all we care about is that _handleMessage is called
       // after the current event loop is unwound and calling the function is
       // a noop when zero requests are pending.
       window.onMessage.listen(this._handleMessage);
   }
 
   void _schedule() {
@@ skipping 72 matching lines @@
 /**
  * Complete all pending microtasks.
  */
 void _completeMicrotasks() {
   var callbacks = _pendingMicrotasks;
   _pendingMicrotasks = null;
   for (var callback in callbacks) {
     callback();
   }
 }