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

tests/html/event_test.dart

 // Copyright (c) 2011, 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.
 
 #library('EventTest');
 #import('../../pkg/unittest/unittest.dart');
 #import('../../pkg/unittest/html_config.dart');
 #import('dart:html');
 
 // TODO(nweiz): Make this private to testEvents when Frog supports closures with
@@ skipping 101 matching lines @@
       // complaining about the types of the named arguments.
       () => new MouseEvent('foo', window, 1, 2, 3, 4, 5, 6, canBubble: true,
           cancelable: true, ctrlKey: true, altKey: true, shiftKey: true,
           metaKey: true, relatedTarget: new Element.tag('div')),
       (ev) {
     Expect.equals(1, ev.detail);
     Expect.equals(2, ev.screenX);
     Expect.equals(3, ev.screenY);
     Expect.equals(4, ev.clientX);
     Expect.equals(5, ev.clientY);
+    Expect.equals(4, ev.offsetX);  // Same as clientX.
+    Expect.equals(5, ev.offsetY);  // Same as clientY.
     Expect.equals(6, ev.button);
     Expect.isTrue(ev.ctrlKey);
     Expect.isTrue(ev.altKey);
     Expect.isTrue(ev.shiftKey);
     Expect.isTrue(ev.metaKey);
     Expect.equals('DIV', ev.relatedTarget.tagName);
   });
 
   eventTest('MutationEvent',
       () => new MutationEvent('foo', new Element.tag('div'), 'red', 'blue',
@@ skipping 82 matching lines @@
     Expect.isTrue(ev.metaKey);
   }, type: 'mousewheel');
 
   // HttpRequestProgressEvent has no properties to itself, so just test that
   // it doesn't error out on creation and can be dispatched.
   // Issue 1005.
   // eventTest('HttpRequestProgressEvent',
   //     () => new HttpRequestProgressEvent('foo', 5),
   //     (ev) {});
 }