Adding .toggle method to Element.classes to match the JS element.classList.toggle method.

lib/dom/templates/html/interface/interface_Element.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.
 
 // WARNING: Do not edit - generated code.
 
 // TODO(vsm): Eliminate this type.
 
 // Note, ElementList implements List (instead of List<Element>) so
 // that its implementing classes may be cast to Lists of more specific
@@ skipping 29 matching lines @@
   ClientRect get bounding();
   // In global coords
   List<ClientRect> get clientRects();
 }
 
 interface NodeSelector {
   Element query(String selectors);
   List<Element> queryAll(String selectors);
 }
 
+interface CSSClassSet extends Set<String> {
+  /**
+   * Adds the class [token] to the element if it is not on it, removes it if it
+   * is.
+   */
+  bool toggle(String token);
+
+  /**
+   * Returns [:true:] classes cannot be added or removed from this
+   * [:CSSClassSet:].
+   */
+  bool get isFrozen();
+}
+
 $!COMMENT
 interface Element extends Node, NodeSelector default _$(ID)FactoryProvider {
   Element.html(String html);
   Element.tag(String tag);
 
   AttributeMap get attributes();
   void set attributes(Map<String, String> value);
 
   /**
    * @domName childElementCount, firstElementChild, lastElementChild,
    *   children, Node.nodes.add
    */
   ElementList get elements();
 
   void set elements(Collection<Element> value);
 
   /** @domName className, classList */
-  Set<String> get classes();
+  CSSClassSet get classes();
 
   void set classes(Collection<String> value);
 
   AttributeMap get dataAttributes();
   void set dataAttributes(Map<String, String> value);
 
   /**
    * @domName getClientRects, getBoundingClientRect, clientHeight, clientWidth,
    * clientTop, clientLeft, offsetHeight, offsetWidth, offsetTop, offsetLeft,
    * scrollHeight, scrollWidth, scrollTop, scrollLeft
    */
   Future<ElementRect> get rect();
 
   /** @domName Window.getComputedStyle */
   Future<CSSStyleDeclaration> get computedStyle();
 
   /** @domName Window.getComputedStyle */
   Future<CSSStyleDeclaration> getComputedStyle(String pseudoElement);
 
   Element clone(bool deep);
 
   Element get parent();
 
 $!MEMBERS
 }