Cherry-picks of multiple html CLs

tools/dom/templates/html/dart2js/impl_MouseEvent.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;
 
 $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
   factory $CLASSNAME(String type,
       {Window view, int detail: 0, int screenX: 0, int screenY: 0,
       int clientX: 0, int clientY: 0, int button: 0, bool canBubble: true,
@@ skipping 50 matching lines @@
   @DomName('MouseEvent.screenY')
   Point get screen => new Point/*<num>*/(_screenX, _screenY);
 
   @DomName('MouseEvent.layerX')
   @DomName('MouseEvent.layerY')
   Point get layer => new Point/*<num>*/(_layerX, _layerY);
 
   @DomName('MouseEvent.pageX')
   @DomName('MouseEvent.pageY')
   Point get page => new Point/*<num>*/(_pageX, _pageY);
+
+  @DomName('MouseEvent.dataTransfer')
+  DataTransfer get dataTransfer => JS('DataTransfer', "#['dataTransfer']", this);
+
 }