dart:html perf optimization based on runing Dromaeo benchmarks

lib/dom/templates/html/dartium/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 $CLASSNAME extends _ElementImpl
-    implements Document {
-$!MEMBERS
-
-  final dom.HTMLDocument _documentPtr;
-  final _NodeImpl _wrappedDocumentPtr;
- 
-_DocumentImpl._wrap(ptr) :
-  super._wrap(ptr),
-  _documentPtr = ptr.parentNode,
-  _wrappedDocumentPtr = ptr.parentNode != null ?
-      new _SecretHtmlDocumentImpl._wrap(ptr.parentNode) : null;
-
-  // For efficiency and simplicity, we always use the HtmlElement as the
-  // Document but sometimes internally we need the real JS document object.
-  _NodeImpl get _rawDocument() => _wrappedDocumentPtr;
-
-  // 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.
-// In a wrapper based world there isn't a need for this complexity but we
-// use this design for consistency with the wrapperless implementation so
-// that bugs show up in both cases.
-class _SecretHtmlDocumentImpl extends _NodeImpl implements Node {
-
-  _SecretHtmlDocumentImpl._wrap(ptr) : super._wrap(ptr);
-
-  _DocumentImpl get _documentElement() => _wrap(_ptr.documentElement);
-}
-
-EventTarget _FixHtmlDocumentReference(EventTarget eventTarget) {
-  if (eventTarget is _SecretHtmlDocumentImpl) {
-    _SecretHtmlDocumentImpl secretDocument = eventTarget;
-    return secretDocument._documentElement;
-  } else {
-    return eventTarget;
-  }
-}