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

lib/html/src/XMLElementWrappingImplementation.dart

 // 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 _XMLClassSet extends _CssClassSet {
   _XMLClassSet(element) : super(element);
 
   String _className() {
     final classStr = _element.getAttribute('class');
     return classStr == null ? '' : classStr;
@@ skipping 11 matching lines @@
 
   factory XMLElementWrappingImplementation.xml(String xml) {
     XMLElement parentTag = new XMLElement.tag('xml');
     parentTag.innerHTML = xml;
     if (parentTag.nodes.length == 1) return parentTag.nodes.removeLast();
 
     throw new IllegalArgumentException(
         'XML had ${parentTag.nodes.length} top-level nodes but 1 expected');
   }
 
-  Set<String> get classes() {
+  CSSClassSet get classes() {
     if (_cssClassSet === null) {
       _cssClassSet = new _XMLClassSet(_ptr);
     }
     return _cssClassSet;
   }
 
   ElementList get elements() {
     if (_elements == null) {
       _elements = new FilteredElementList(this);
     }
@@ skipping 139 matching lines @@
 
   void set lang(String value) { attributes['lang'] = value; }
 
   String get dir() => _attr('dir');
 
   void set dir(String value) { attributes['dir'] = value; }
 
   String _attr(String name, [String def = '']) =>
     attributes.containsKey(name) ? attributes[name] : def;
 }