Changing CSSClassSet from isFrozen to frozen for API consistency.

lib/html/dartium/html_dartium.dart

Index: lib/html/dartium/html_dartium.dart
diff --git a/lib/html/dartium/html_dartium.dart b/lib/html/dartium/html_dartium.dart
index dc8db31d9c58e5176022cbd72a00463d705c7494..89c35d1ac17a7eca61b4e8e00abfb509cd0f462d 100644
--- a/lib/html/dartium/html_dartium.dart
+++ b/lib/html/dartium/html_dartium.dart
@@ -11752,7 +11752,7 @@ class _FrozenCSSClassSet extends _CssClassSet {
   }
   Set<String> _read() => new Set<String>();
 
-  bool get isFrozen => true;
+  bool get frozen => true;
 }
 
 class _DocumentFragmentImpl extends _NodeImpl implements DocumentFragment {
@@ -12433,7 +12433,7 @@ abstract class CSSClassSet implements Set<String> {
    * Returns [:true:] classes cannot be added or removed from this
    * [:CSSClassSet:].
    */
-  bool get isFrozen;
+  bool get frozen;
 }
 
 /// @domName Element
@@ -13376,7 +13376,7 @@ class _CssClassSet implements CSSClassSet {
 
   bool isEmpty() => _read().isEmpty();
 
-  bool get isFrozen => false;
+  bool get frozen => false;
 
   int get length =>_read().length;