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

lib/dom/scripts/systemhtml.py

 #!/usr/bin/python
 # 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.
 
 """This module provides shared functionality for the system to generate
 Dart:html APIs from the IDL database."""
 
 import emitter
 
 from systemdart2js import *
 from systeminterface import *
 
 _js_custom_members = set([
     'IDBDatabase.transaction',
     'IFrameElement.contentWindow',
+    'MouseEvent.offsetX',
+    'MouseEvent.offsetY',
     'Window.document',
     'Window.top',
     'Window.location',
     'Window.open',
     ])
 
 # This map controls merging of interfaces in dart:html library.
 # All constants, attributes, and operations of merged interface (key) are
 # added to target interface (value). All references to the merged interface
 # (e.g. parameter types, return types, parent interfaces) are replaced with
@@ skipping 1198 matching lines @@
     return HtmlDart2JSClassGenerator(self, interface)
 
   def GenerateLibraries(self, dart_files):
     self._GenerateLibFile(
         'html_dart2js.darttemplate',
         os.path.join(self._output_dir, 'html_dart2js.dart'),
         dart_files)
 
   def Finish(self):
     pass