dart:html perf optimization based on runing Dromaeo benchmarks

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:dom', prefix:'dom');
@@ skipping 14 matching lines @@
 #source('../../html/src/_Testing.dart');
 #source('../../html/src/_DOMTypeBase.dart');
 #source('../../dom/src/_ListIterators.dart');
 #source('../../dom/src/_Lists.dart');
 
 _WindowImpl __window;
 _DocumentImpl __document;
 
 void _initialize() {
   __window = _wrap(dom.window);
-  __document = _wrap(dom.document.documentElement);
+  __document = _wrap(dom.document);
 }
 
 Window get window() {
   if (__window == null) {
     _initialize();
   }
   return __window;
 }
 
 Document get document() {
@@ skipping 21 matching lines @@
   return raw is _DOMTypeBase ? raw._ptr : raw;
 }
 
 // Warning: does not attempt wrap event listeners.
 _wrap(raw) {
   if (raw is! dom.DOMType) return raw;
   dom.DOMType domObject = raw;
   if (domObject.dartObjectLocalStorage != null)
     return domObject.dartObjectLocalStorage;
   switch(domObject.typeName) {
-    case 'HTMLDocument':
-      throw 'A document should never be wrapped directly. TODO(jacobr) XXX';
-    case 'HTMLHtmlElement':
-      return new _DocumentImpl._wrap(domObject);
     case 'HTMLElement':
       return new _UnknownElementImpl._wrap(domObject);
 $WRAPCASES
+    case 'HTMLDocument':
+      return new _DocumentImpl._wrap(domObject);
     default:
       throw 'Unrecognized object $domObject. Name=${domObject.typeName}';
   }
 }