dart:html perf optimization based on runing Dromaeo benchmarks

lib/dom/templates/html/impl/impl_DocumentFragment.darttemplate

 // Copyright (c) 2011, 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 FilteredElementList implements ElementList {
   final Node _node;
   final NodeList _childNodes;
 
   FilteredElementList(Node node): _childNodes = node.nodes, _node = node;
 
@@ skipping 128 matching lines @@
   // TODO: The type of value should be Collection<Element>. See http://b/5392897
   void set elements(value) {
     // Copy list first since we don't want liveness during iteration.
     List copy = new List.from(value);
     final elements = this.elements;
     elements.clear();
     elements.addAll(copy);
   }
 
   ElementList queryAll(String selectors) =>
-    new _FrozenElementList._wrap(_querySelectorAll(selectors));
+    new _FrozenElementList._wrap($dom_querySelectorAll(selectors));
 
   String get innerHTML() {
     final e = new Element.tag("div");
     e.nodes.add(this.clone(true));
     return e.innerHTML;
   }
 
   String get outerHTML() => innerHTML;
 
   // TODO(nweiz): Do we want to support some variant of innerHTML for XML and/or
@@ skipping 48 matching lines @@
   bool get draggable() => false;
   bool get hidden() => false;
   bool get spellcheck() => false;
   bool get translate() => false;
   int get tabIndex() => -1;
   String get id() => "";
   String get title() => "";
   String get tagName() => "";
   String get webkitdropzone() => "";
   String get webkitRegionOverflow() => "";
-  Element get firstElementChild() => elements.first();
-  Element get lastElementChild() => elements.last();
+  Element get $dom_firstElementChild() => elements.first();
+  Element get $dom_lastElementChild() => elements.last();
   Element get nextElementSibling() => null;
   Element get previousElementSibling() => null;
   Element get offsetParent() => null;
   Element get parent() => null;
   Map<String, String> get attributes() => const {};
   // Issue 174: this should be a const set.
   Set<String> get classes() => new Set<String>();
   Map<String, String> get dataAttributes() => const {};
   CSSStyleDeclaration get style() => new Element.tag('div').style;
   Future<CSSStyleDeclaration> get computedStyle() =>
@@ skipping 105 matching lines @@
       "WebKit drop zone can't be set for document fragments.");
   }
 
   void set webkitRegionOverflow(String value) {
     throw new UnsupportedOperationException(
       "WebKit region overflow can't be set for document fragments.");
   }
 
 $!MEMBERS
 }