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

lib/dom/templates/html/impl/impl_Element.darttemplate

 // 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.
 
 // TODO(jacobr): use _Lists.dart to remove some of the duplicated
 // functionality.
 class _ChildrenElementList implements ElementList {
   // Raw Element.
   final _ElementImpl _element;
   final _HTMLCollectionImpl _childElements;
@@ skipping 452 matching lines @@
 
   // TODO: Use lazy iterator when it is available on Map.
   bool isEmpty() => length == 0;
 
   // Helpers.
   String _attr(String key) => 'data-$key';
   bool _matches(String key) => key.startsWith('data-');
   String _strip(String key) => key.substring(5);
 }
 
-class _CssClassSet implements Set<String> {
+class _CssClassSet implements CSSClassSet {
 
   final _ElementImpl _element;
 
   _CssClassSet(this._element);
 
   String toString() => _formatSet(_read());
 
   // interface Iterable - BEGIN
   Iterator<String> iterator() => _read().iterator();
   // interface Iterable - END
 
   // interface Collection - BEGIN
   void forEach(void f(String element)) {
     _read().forEach(f);
   }
 
   Collection map(f(String element)) => _read().map(f);
 
   Collection<String> filter(bool f(String element)) => _read().filter(f);
 
   bool every(bool f(String element)) => _read().every(f);
 
   bool some(bool f(String element)) => _read().some(f);
 
   bool isEmpty() => _read().isEmpty();
 
+  bool get isFrozen() => false;
+
   int get length() =>_read().length;
 
   // interface Collection - END
 
   // interface Set - BEGIN
   bool contains(String value) => _read().contains(value);
 
   void add(String value) {
     // TODO - figure out if we need to do any validation here
     // or if the browser natively does enough
     _modify((s) => s.add(value));
   }
 
   bool remove(String value) {
     Set<String> s = _read();
     bool result = s.remove(value);
     _write(s);
     return result;
   }
 
+  bool toggle(String value) {
+    Set<String> s = _read();
+    bool result = false;
+    if (s.contains(value)) {
+      s.remove(value);
+    } else {
+      s.add(value);
+      result = true;
+    }
+    _write(s);
+    return result;
+  }
+
   void addAll(Collection<String> collection) {
     // TODO - see comment above about validation
     _modify((s) => s.addAll(collection));
   }
 
   void removeAll(Collection<String> collection) {
     _modify((s) => s.removeAll(collection));
   }
 
   bool isSubsetOf(Collection<String> collection) =>
@@ skipping 87 matching lines @@
  * All your element measurement needs in one place
  * @domName none
  */
 class _ElementRectImpl implements ElementRect {
   final ClientRect client;
   final ClientRect offset;
   final ClientRect scroll;
 
   // TODO(jacobr): should we move these outside of ElementRect to avoid the
   // overhead of computing them every time even though they are rarely used.
-  final _ClientRectImpl _boundingClientRect; 
+  final _ClientRectImpl _boundingClientRect;
   final _ClientRectListImpl _clientRects;
 
   _ElementRectImpl(_ElementImpl element) :
     client = new _SimpleClientRect(element.$dom_clientLeft,
                                   element.$dom_clientTop,
-                                  element.$dom_clientWidth, 
-                                  element.$dom_clientHeight), 
+                                  element.$dom_clientWidth,
+                                  element.$dom_clientHeight),
     offset = new _SimpleClientRect(element.$dom_offsetLeft,
                                   element.$dom_offsetTop,
                                   element.$dom_offsetWidth,
                                   element.$dom_offsetHeight),
     scroll = new _SimpleClientRect(element.$dom_scrollLeft,
                                   element.$dom_scrollTop,
                                   element.$dom_scrollWidth,
                                   element.$dom_scrollHeight),
     _boundingClientRect = element.$dom_getBoundingClientRect(),
     _clientRects = element.$dom_getClientRects();
@@ skipping 150 matching lines @@
 
   /** @domName Document.createElement */
 $if DART2JS
   // Optimization to improve performance until the dart2js compiler inlines this
   // method.
   factory Element.tag(String tag) native "return document.createElement(tag)";
 $else
   factory Element.tag(String tag) => _document.$dom_createElement(tag);
 $endif
 }