dom: more final more of the time

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
  * unwinds, but before the browser repaints.
  */
 abstract class _MicrotaskScheduler {
   bool _nextMicrotaskFrameScheduled = false;
-  _MicrotaskCallback _callback;
+  final _MicrotaskCallback _callback;
 
   _MicrotaskScheduler(this._callback);
 
   /**
    * Creates the best possible microtask scheduler for the current platform.
    */
   factory _MicrotaskScheduler.best(_MicrotaskCallback callback) {
     if (Window._supportsSetImmediate) {
       return new _SetImmediateScheduler(callback);
     } else if (MutationObserver.supported) {
@@ skipping 121 matching lines @@
 /**
  * Complete all pending microtasks.
  */
 void _completeMicrotasks() {
   var callbacks = _pendingMicrotasks;
   _pendingMicrotasks = null;
   for (var callback in callbacks) {
     callback();
   }
 }