Fix MouseEvent.movement to not use old webkit prefix

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 11 matching lines @@
   }
 $!MEMBERS
 
   @DomName('MouseEvent.clientX')
   @DomName('MouseEvent.clientY')
   Point get client => new Point(_clientX, _clientY);
 
   @DomName('MouseEvent.movementX')
   @DomName('MouseEvent.movementY')
   @SupportedBrowser(SupportedBrowser.CHROME)
-  @SupportedBrowser(SupportedBrowser.SAFARI)
+  @SupportedBrowser(SupportedBrowser.FIREFOX)
   @Experimental()
-  Point get movement => new Point(_webkitMovementX, _webkitMovementY);
+  Point get movement => new Point(movementX, movementY);
 
   /**
    * The coordinates of the mouse pointer in target node coordinates.
    *
    * This value may vary between platforms if the target node moves
    * after the event has fired or if the element has CSS transforms affecting
    * it.
    */
   Point get offset {
     if (JS('bool', '!!#.offsetX', this)) {
@@ skipping 17 matching lines @@
   Point get screen => new Point(_screenX, _screenY);
 
   @DomName('MouseEvent.layerX')
   @DomName('MouseEvent.layerY')
   Point get layer => new Point(_layerX, _layerY);
 
   @DomName('MouseEvent.pageX')
   @DomName('MouseEvent.pageY')
   Point get page => new Point(_pageX, _pageY);
 }