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

lib/html/src/SVGElementWrappingImplementation.dart

 // 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.
 
 class _SVGClassSet extends _CssClassSet {
   _SVGClassSet(element) : super(element);
 
   String _className() => _element.className.baseVal;
 
   void _write(Set s) {
@@ skipping 18 matching lines @@
     }
 
     parentTag.innerHTML = svg;
     if (parentTag.elements.length == 1) return parentTag.nodes.removeLast();
 
     throw new IllegalArgumentException(
         'SVG had ${parentTag.elements.length} '
         'top-level elements but 1 expected');
   }
 
-  Set<String> get classes() {
+  CSSClassSet get classes() {
     if (_cssClassSet === null) {
       _cssClassSet = new _SVGClassSet(_ptr);
     }
     return _cssClassSet;
   }
 
   String get id() { return _ptr.id; }
 
   void set id(String value) { _ptr.id = value; }
 
@@ skipping 33 matching lines @@
   void set innerHTML(String svg) {
     var container = new Element.tag("div");
     // Wrap the SVG string in <svg> so that SVGElements are created, rather than
     // HTMLElements.
     container.innerHTML = '<svg version="1.1">$svg</svg>';
     this.elements = container.elements.first.elements;
   }
 
   SVGElement clone(bool deep) => super.clone(deep);
 }