Fixing Window.animationFrame timing issue

tools/dom/templates/html/impl/impl_Window.darttemplate

 // 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 $LIBRARYNAME;
 
 @DocsEditable()
 $if DART2JS
 $(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS native "Window,DOMWindow" {
 $else
 $(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $endif
 
   /**
    * Returns a Future that completes just before the window is about to
    * repaint so the user can draw an animation frame.
    *
    * If you need to later cancel this animation, use [requestAnimationFrame]
    * instead.
    *
    * The [Future] completes to a timestamp that represents a floating
    * point value of the number of milliseconds that have elapsed since the page
    * started to load (which is also the timestamp at this call to
    * animationFrame).
    *
    * Note: The code that runs when the future completes should call
    * [animationFrame] again for the animation to continue.
    */
   Future<num> get animationFrame {
-    var completer = new Completer<num>();
+    var completer = new Completer<num>.sync();
     requestAnimationFrame((time) {
       completer.complete(time);
     });
     return completer.future;
   }
 
 $if DART2JS
   /**
    * The newest document in this window.
    *
@@ skipping 347 matching lines @@
     return controller.stream;
 $else
     return stream;
 $endif
   }
 
   String getEventType(EventTarget target) {
     return _eventType;
   }
 }