Use fields in hidden native DOM classes instead of getters/setters.

client/dom/generated/src/frog/Storage.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 _StorageJs extends _DOMTypeJs implements Storage native "*Storage" {
 
-  int get length() native "return this.length;";
+  final int length;
 
   void clear() native;
 
   String getItem(String key) native;
 
   String key(int index) native;
 
   void removeItem(String key) native;
 
   void setItem(String key, String data) native;
@@ skipping 14 matching lines @@
 
   void set dartObjectLocalStorage(var value) native """
     if (this === window.localStorage)
       window._dartLocalStorageLocalStorage = value;
     else if (this === window.sessionStorage)
       window._dartSessionStorageLocalStorage = value;
     else
       throw new UnsupportedOperationException('Cannot dartObjectLocalStorage for unknown Storage object.');
 """ { throw new UnsupportedOperationException(''); }
 }