Fix checked mode errors in previous commit

sdk/lib/io/directory_impl.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.
 
 part of dart.io;
 
 class _Directory implements Directory {
   static const CREATE_REQUEST = 0;
   static const DELETE_REQUEST = 1;
   static const EXISTS_REQUEST = 2;
   static const CREATE_TEMP_REQUEST = 3;
   static const LIST_REQUEST = 4;
   static const RENAME_REQUEST = 5;
 
   static const SUCCESS_RESPONSE = 0;
   static const ILLEGAL_ARGUMENT_RESPONSE = 1;
   static const OSERROR_RESPONSE = 2;
 
   _Directory(String this._path);
   _Directory.fromPath(Path path) : this(path.toNativePath());
 
   external static String _current();
-  external static _setCurrent(String path);
+  external static _setCurrent(path);
   external static _createTemp(String template);
   external static int _exists(String path);
   external static _create(String path);
   external static _delete(String path, bool recursive);
   external static _rename(String path, String newPath);
   external static List _list(String path, bool recursive, bool followLinks);
   external static SendPort _newServicePort();
 
   static Directory get current => new _Directory(_current());
 
@@ skipping 301 matching lines @@
 
   void _ensureDirectoryService() {
     if (_directoryService == null) {
       _directoryService = _newServicePort();
     }
   }
 
   final String _path;
   SendPort _directoryService;
 }