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

lib/html/src/XMLDocumentWrappingImplementation.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 XMLDocumentWrappingImplementation extends DocumentWrappingImplementation
     implements XMLDocument {
   // This really just wants to extend both DocumentWrappingImplementation and
   // XMLElementWrappingImplementation, but since that's not possible we delegate
   // to the latter.
   XMLElement documentEl;
@@ skipping 95 matching lines @@
         "XML documents don't support cookies.");
   }
 
   String get manifest() => "";
 
   void set manifest(String value) {
     throw new UnsupportedOperationException(
         "Manifest can't be set for XML documents.");
   }
 
-  Set<String> get classes() => documentEl.classes;
+  CSSClassSet get classes() => documentEl.classes;
 
   ElementList get elements() => documentEl.elements;
 
   void set elements(Collection<Element> value) { documentEl.elements = value; }
 
   String get outerHTML() => documentEl.outerHTML;
 
   String get innerHTML() => documentEl.innerHTML;
 
   void set innerHTML(String xml) { documentEl.innerHTML = xml; }
@@ skipping 33 matching lines @@
   void set webkitdropzone(String value) { documentEl.webkitdropzone = value; }
 
   String get lang() => documentEl.lang;
 
   void set lang(String value) { documentEl.lang = value; }
 
   String get dir() => documentEl.dir;
 
   void set dir(String value) { documentEl.dir = value; }
 }