Minimal fix to get cross frame tests working, take #2.

lib/dom/templates/html/dartium/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.
 
 class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 
   _DocumentImpl get document() => _wrap(_ptr.document);
 
   void requestLayoutFrame(TimeoutHandler callback) {
     _addMeasurementFrameCallback(callback);
   }
 
   int requestAnimationFrame(RequestAnimationFrameCallback callback) =>
       webkitRequestAnimationFrame(callback);
 
+  Window get top() => _wrap(_ptr.top);
+
 $!MEMBERS
 }