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

lib/dom/templates/html/impl/impl_DocumentFragment.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.
 
 class FilteredElementList implements ElementList {
   final Node _node;
   final NodeList _childNodes;
 
   FilteredElementList(Node node): _childNodes = node.nodes, _node = node;
 
@@ skipping 108 matching lines @@
 class EmptyElementRect implements ElementRect {
   final ClientRect client = const _SimpleClientRect(0, 0, 0, 0);
   final ClientRect offset = const _SimpleClientRect(0, 0, 0, 0);
   final ClientRect scroll = const _SimpleClientRect(0, 0, 0, 0);
   final ClientRect bounding = const _SimpleClientRect(0, 0, 0, 0);
   final List<ClientRect> clientRects = const <ClientRect>[];
 
   const EmptyElementRect();
 }
 
+class _FrozenCSSClassSet extends _CssClassSet {
+  _FrozenCSSClassSet() : super(null);
+
+  void _write(Set s) {
+    throw const UnsupportedOperationException(
+        'frozen class set cannot be modified');
+  }
+  Set<String> _read() => new Set<String>();
+}
+
 class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   ElementList _elements;
 
   ElementList get elements() {
     if (_elements == null) {
       _elements = new FilteredElementList(this);
     }
     return _elements;
   }
 
@@ skipping 78 matching lines @@
   String get tagName() => "";
   String get webkitdropzone() => "";
   String get webkitRegionOverflow() => "";
   Element get $dom_firstElementChild() => elements.first();
   Element get $dom_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() => new _FrozenCSSClassSet();
   Map<String, String> get dataAttributes() => const {};
   CSSStyleDeclaration get style() => new Element.tag('div').style;
   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 98 matching lines @@
       "WebKit drop zone can't be set for document fragments.");
   }
 
   void set webkitRegionOverflow(String value) {
     throw new UnsupportedOperationException(
       "WebKit region overflow can't be set for document fragments.");
   }
 
 $!MEMBERS
 }