dart:html perf optimization based on runing Dromaeo benchmarks

lib/dom/templates/html/frog/impl_Document.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.
-
-class $CLASSNAME extends _ElementImpl
-    implements Document
-    native "*HTMLHtmlElement" {
-$!MEMBERS
-
-  // For efficiency and simplicity, we always use the HtmlElement as the
-  // Document but sometimes internally we need the real JS document object.
-  _NodeImpl get _jsDocument() native "return this.parentNode;";
-
-  // The document doesn't have a parent element.
-  _ElementImpl get parent() => null;
-}
-
-// This class should not be externally visible.  If a user ever gets access to
-// a _SecretHtmlDocumentImpl object that is a bug.  This object is hidden by
-// adding checks to all methods that could an HTMLDocument.  We believe that
-// list is limited to Event.target, and HTMLHtmlElement.parent.
-class _SecretHtmlDocumentImpl extends _NodeImpl implements Node
-    native "*HTMLDocument" {
-  _DocumentImpl get _documentElement() native "return this.documentElement;";
-}
-
-EventTarget _FixHtmlDocumentReference(EventTarget eventTarget) {
-  if (eventTarget is _SecretHtmlDocumentImpl) {
-    _SecretHtmlDocumentImpl secretDocument = eventTarget;
-    return secretDocument._documentElement;
-  } else {
-    return eventTarget;
-  }
-}