Use 'field' syntax in dart:dom interfaces.

client/dom/generated/src/interface/DOMTokenList.dart

 // 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.
 
 // WARNING: Do not edit - generated code.
 
 interface DOMTokenList {
 
-  int get length();
+  final int length;
 
   void add(String token);
 
   bool contains(String token);
 
   String item(int index);
 
   void remove(String token);
 
   String toString();
 
   bool toggle(String token);
 }