dart:html perf optimization based on runing Dromaeo benchmarks

lib/dom/templates/html/frog/html_frog.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");
 
 $!GENERATED_DART_FILES
 
 #source('../../dom/src/EventListener.dart');
 #source('../../dom/src/KeyLocation.dart');
 #source('../../dom/src/KeyName.dart');
 #source('../../dom/src/ReadyState.dart');
 #source('../../dom/src/TimeoutHandler.dart');
 #source('../../dom/src/_Collections.dart');
 #source('../../dom/src/_XMLHttpRequestUtils.dart');
 #source('../../html/src/Measurement.dart');
 #source('../../html/src/shared_FactoryProviders.dart');
 #source('../../html/src/frog_FactoryProviders.dart');
 #source('../../html/src/_Testing.dart');
 #source('../../html/src/Device.dart');
 #source('../../dom/src/_ListIterators.dart');
 #source('../../dom/src/_Lists.dart');
 
-_WindowImpl _cachedWindow;
-_DocumentImpl _cachedDocument;
 
-void _init() {
-  _cachedDocument = _document;
-  _cachedWindow = _window;
-  // Feature detect that dart:dom and dart:html are not both loaded by
-  // checking for the presence of a bug that manifests itself when both
-  // libraries are loaded.
-  // TODO(jacobr): remove this code once b/1911 is fixed and the frog compiler
-  // is changed to generate compile time errors if two libraries that define
-  // the same native types in conflicting ways are imported.
-  var element = new Element.tag('body');
-  element.innerHTML = 'f';
-  if (element.text == '') {
-    _cachedWindow.console.error(
-      'Cannot import dart:html and dart:dom within the same application.');
-    throw new UnsupportedOperationException(
-      'Cannot import dart:html and dart:dom within the same application.');
-  }
-}
-
-Window get window() {
-  if (_cachedWindow == null) {
-    _init();
-  }
-  return _cachedWindow;
-}
-
+Window get window() native "return window;";
 _WindowImpl get _window() native "return window;";
 
-Document get document() {
-  if (_cachedDocument == null) {
-    _init();
-  }
-  return _cachedDocument;
-}
+Document get document() native "return document;";
 
-_DocumentImpl get _document() native "return window.document.documentElement;";
+_DocumentImpl get _document() native "return document;";
 
 // Workaround for tags like <cite> that lack their own Element subclass --
 // Dart issue 1990.
 class _HTMLElementImpl extends _ElementImpl native "*HTMLElement" {
 }