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

lib/dom/templates/html/interface/interface_Element.darttemplate

Index: lib/dom/templates/html/interface/interface_Element.darttemplate
diff --git a/lib/dom/templates/html/interface/interface_Element.darttemplate b/lib/dom/templates/html/interface/interface_Element.darttemplate
index 455eb0d24f804990c5402ad2773a97dca3bd0125..16d2279065b24f250ef037a5569464211f8f2f87 100644
--- a/lib/dom/templates/html/interface/interface_Element.darttemplate
+++ b/lib/dom/templates/html/interface/interface_Element.darttemplate
@@ -47,6 +47,20 @@ interface NodeSelector {
   List<Element> queryAll(String selectors);
 }
 
+interface CSSClassSet extends Set<String> {
+  /**
+   * Adds the class [token] to the element if it is not on it, removes it if it
+   * is.
+   */
+  bool toggle(String token);
+
+  /**
+   * Returns [:true:] classes cannot be added or removed from this
+   * [:CSSClassSet:].
+   */
+  bool get isFrozen();
+}
+
 $!COMMENT
 interface Element extends Node, NodeSelector default _$(ID)FactoryProvider {
   Element.html(String html);
@@ -64,7 +78,7 @@ interface Element extends Node, NodeSelector default _$(ID)FactoryProvider {
   void set elements(Collection<Element> value);
 
   /** @domName className, classList */
-  Set<String> get classes();
+  CSSClassSet get classes();
 
   void set classes(Collection<String> value);