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

tests/html/documentfragment_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('DocumentFragmentTest');
 #import('../../pkg/unittest/unittest.dart');
 #import('../../pkg/unittest/html_config.dart');
 #import('dart:html');
 #source('util.dart');
 
@@ skipping 63 matching lines @@
     // test('.xml parses input as XML', () {
     //   final fragment = new DocumentFragment.xml('<a>foo</a>');
     //   Expect.isTrue(fragment.elements.first is XMLElement);
     // });
   });
 
   test('Unsupported operations throw errors', () {
     var emptyFragment = new DocumentFragment();
     expectUnsupported(() => emptyFragment.attributes = {});
     expectUnsupported(() => emptyFragment.classes = []);
+    expectUnsupported(() => emptyFragment.classes.add('foo'));
     expectUnsupported(() => emptyFragment.dataAttributes = {});
     expectUnsupported(() => emptyFragment.contentEditable = "true");
     expectUnsupported(() => emptyFragment.dir);
     expectUnsupported(() => emptyFragment.dir = "ltr");
     expectUnsupported(() => emptyFragment.draggable = true);
     expectUnsupported(() => emptyFragment.hidden = true);
     expectUnsupported(() => emptyFragment.id = "foo");
     expectUnsupported(() => emptyFragment.lang);
     expectUnsupported(() => emptyFragment.lang = "en");
     expectUnsupported(() => emptyFragment.scrollLeft = 10);
@@ skipping 250 matching lines @@
   //   // assertConstError(() => fragment.classes.add('foo'));
   // });
 
   test('query searches the fragment', () {
     var fragment = new DocumentFragment.html(
       "<div class='foo'><a>foo</a><b>bar</b></div>");
     Expect.equals("A", fragment.query(".foo a").tagName);
     Expect.listEquals(["A", "B"], _nodeStrings(fragment.queryAll(".foo *")));
   });
 }