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

lib/html/src/DocumentFragmentWrappingImplementation.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.
 
 class FilteredElementList implements ElementList {
   final Node _node;
   final NodeList _childNodes;
 
   FilteredElementList(Node node): _childNodes = node.nodes, _node = node;
 
@@ skipping 270 matching lines @@
   String get title() => "";
   String get tagName() => "";
   String get webkitdropzone() => "";
   Element get firstElementChild() => elements.first();
   Element get 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>();
+  CSSClassSet get classes() => null;
   Map<String, String> get dataAttributes() => const {};
   CSSStyleDeclaration get style() => new EmptyStyleDeclaration();
   Future<CSSStyleDeclaration> get computedStyle() =>
       _emptyStyleFuture();
   Future<CSSStyleDeclaration> getComputedStyle(String pseudoElement) =>
       _emptyStyleFuture();
   bool matchesSelector([String selectors]) => false;
 
   // Imperative Element methods are made into no-ops, as they are on parentless
   // elements.
@@ skipping 85 matching lines @@
       "Title can't be set for document fragments.");
   }
 
   void set webkitdropzone(String value) {
     throw new UnsupportedOperationException(
       "WebKit drop zone can't be set for document fragments.");
   }
 
   DocumentFragment clone(bool deep) => super.clone(deep);
 }