Adding polyfills for MouseEvent.offsetX and MouseEvent.offsetY so they work on FF.

lib/html/doc/interface/MouseEvent.dartdoc

 // 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.
 
 // WARNING:
 //   This file contains documentation that is merged into the real source.
 //   Do not make code changes here.
 
 /// @domName MouseEvent
 interface MouseEvent extends UIEvent default _MouseEventFactoryProvider {
@@ skipping 21 matching lines @@
 
   /** @domName MouseEvent.dataTransfer */
   final Clipboard dataTransfer;
 
   /** @domName MouseEvent.fromElement */
   final Node fromElement;
 
   /** @domName MouseEvent.metaKey */
   final bool metaKey;
 
-  /** @domName MouseEvent.offsetX */
+  /**
+   * The X coordinate 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.
+   */
   final int offsetX;
 
-  /** @domName MouseEvent.offsetY */
+  /**
+   * The Y coordinate 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.
+   */
   final int offsetY;
 
   /** @domName MouseEvent.relatedTarget */
   final EventTarget relatedTarget;
 
   /** @domName MouseEvent.screenX */
   final int screenX;
 
   /** @domName MouseEvent.screenY */
   final int screenY;
@@ skipping 12 matching lines @@
 
   /** @domName MouseEvent.x */
   final int x;
 
   /** @domName MouseEvent.y */
   final int y;
 
   /** @domName MouseEvent.initMouseEvent */
   void $dom_initMouseEvent(String type, bool canBubble, bool cancelable, Window view, int detail, int screenX, int screenY, int clientX, int clientY, bool ctrlKey, bool altKey, bool shiftKey, bool metaKey, int button, EventTarget relatedTarget);
 }