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

tests/html/element_classes_test.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.
 
 #library('ElementTest');
 #import('../../pkg/unittest/unittest.dart');
 #import('../../pkg/unittest/html_config.dart');
 #import('dart:html');
 
 main() {
@@ skipping 105 matching lines @@
   });
 
   test('remove', () {
     final classes = makeClassSet();
     classes.remove('bar');
     Expect.setEquals(['foo', 'baz'], classes);
     classes.remove('qux');
     Expect.setEquals(['foo', 'baz'], classes);
   });
 
+  test('toggle', () {
+    final classes = makeClassSet();
+    classes.toggle('bar');
+    Expect.setEquals(['foo', 'baz'], classes);
+    classes.toggle('qux');
+    Expect.setEquals(['foo', 'baz', 'qux'], classes);
+  });
+
   test('addAll', () {
     final classes = makeClassSet();
     classes.addAll(['bar', 'qux', 'bip']);
     Expect.setEquals(['foo', 'bar', 'baz', 'qux', 'bip'], classes);
   });
 
   test('removeAll', () {
     final classes = makeClassSet();
     classes.removeAll(['bar', 'baz', 'qux']);
     Expect.setEquals(['foo'], classes);
@@ skipping 17 matching lines @@
     Expect.setEquals(['foo', 'baz'],
         classes.intersection(['foo', 'qux', 'baz']));
   });
 
   test('clear', () {
     final classes = makeClassSet();
     classes.clear();
     Expect.setEquals([], classes);
   });
 }