Fix dart to native dispatch.

lib/dom/templates/html/dartium/html_dartium.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.
 
 // DO NOT EDIT
 // Auto-generated dart:html library.
 
 #library("html");
 
 #import("dart:isolate");
@@ skipping 36 matching lines @@
 
 Document get document() {
   if (__document !== null) {
     return __document;
   }
   __document = _document;
   return __document;
 }
 
 Document get _document() => _window.document;
+
+class _Null {
+  const _Null();
+}
+
+final _null = const _Null();