Changing CSSClassSet from isFrozen to frozen for API consistency.

lib/html/templates/html/impl/impl_DocumentFragment.darttemplate

 // 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.
 
 class FilteredElementList implements ElementList {
   final Node _node;
   final NodeList _childNodes;
 
   FilteredElementList(Node node): _childNodes = node.nodes, _node = node;
 
@@ skipping 113 matching lines @@
 
 class _FrozenCSSClassSet extends _CssClassSet {
   _FrozenCSSClassSet() : super(null);
 
   void _write(Set s) {
     throw const UnsupportedOperationException(
         'frozen class set cannot be modified');
   }
   Set<String> _read() => new Set<String>();
 
-  bool get isFrozen => true;
+  bool get frozen => true;
 }
 
 class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   ElementList _elements;
 
   ElementList get elements {
     if (_elements == null) {
       _elements = new FilteredElementList(this);
     }
     return _elements;
@@ skipping 207 matching lines @@
       "WebKit drop zone can't be set for document fragments.");
   }
 
   void set webkitRegionOverflow(String value) {
     throw new UnsupportedOperationException(
       "WebKit region overflow can't be set for document fragments.");
   }
 
 $!MEMBERS
 }