Make DocumentFragment implement Element.

client/html/src/Element.dart

 // 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.
 
 interface ElementList extends List<Element> {
   // TODO(jacobr): add element batch manipulation methods.
   Element get first();
   // TODO(jacobr): add insertAt
 }
 
@@ skipping 65 matching lines @@
   Element.tag(String tag);
 
   Map<String, String> get attributes();
   void set attributes(Map<String, String> value);
 
   ElementList get elements();
 
   // TODO: The type of value should be Collection<Element>. See http://b/5392897
   void set elements(value);
 
-  _CssClassSet get classes();
+  Set<String> get classes();
 
   // TODO: The type of value should be Collection<String>. See http://b/5392897
   void set classes(value);
 
   Map<String, String> get dataAttributes();
   void set dataAttributes(Map<String, String> value);
 
   int get clientHeight();
 
   int get clientLeft();
@@ skipping 83 matching lines @@
   String get webkitdropzone();
 
   void set webkitdropzone(String value);
 
   void blur();
 
   void focus();
 
   ClientRect getBoundingClientRect();
 
-  ClientRectList getClientRects();
+  List<ClientRect> getClientRects();
 
   Element insertAdjacentElement([String where, Element element]);
 
   void insertAdjacentHTML([String position_OR_where, String text]);
 
   void insertAdjacentText([String where, String text]);
 
   Element query(String selectors);
 
   ElementList queryAll(String selectors);
 
   Element get parent();
 
   void scrollByLines([int lines]);
 
   void scrollByPages([int pages]);
 
   void scrollIntoView([bool centerIfNeeded]);
 
   bool matchesSelector([String selectors]);
 
   ElementEvents get on();
 }