Refresh dom_frog.

client/dom/generated/src/wrapping/_DataTransferItemWrappingImplementation.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.
 
 class _DataTransferItemWrappingImplementation extends DOMWrapperBase implements DataTransferItem {
   _DataTransferItemWrappingImplementation() : super() {}
 
   static create__DataTransferItemWrappingImplementation() native {
     return new _DataTransferItemWrappingImplementation();
   }
 
   String get kind() { return _get_kind(this); }
   static String _get_kind(var _this) native;
 
   String get type() { return _get_type(this); }
   static String _get_type(var _this) native;
 
   Blob getAsFile() {
     return _getAsFile(this);
   }
   static Blob _getAsFile(receiver) native;
 
-  void getAsString(StringCallback callback) {
+  void getAsString([StringCallback callback = null]) {
     _getAsString(this, callback);
     return;
   }
   static void _getAsString(receiver, callback) native;
 
   String get typeName() { return "DataTransferItem"; }
 }