Fix typo (offsetX instead of offsetY).

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)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   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 42 matching lines @@
   /**
    * 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)) {
       var x = JS('int', '#.offsetX', this);
-      var y = JS('int', '#.offsetX', this);
+      var y = JS('int', '#.offsetY', this);
       return new Point(x, y);
     } else {
       // Firefox does not support offsetX.
       var target = this.target;
       if (!(target is Element)) {
         throw new UnsupportedError(
             'offsetX is only supported on elements');
       }
       return (this.client -
           this.target.getBoundingClientRect().topLeft).toInt();
     }
   }
 
   @DomName('MouseEvent.screenX')
   @DomName('MouseEvent.screenY')
   Point get screen => new Point($dom_screenX, $dom_screenY);
 }