Fix type annotation error in directory implementation.

runtime/bin/directory_impl.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.
 
 
 class DirectoryException {
   String toString() { return "DirectoryException: $message"; }
   const DirectoryException(String this.message);
   final String message;
 }
@@ skipping 126 matching lines @@
   }
 
   void _closePort(ReceivePort port) {
     if (port !== null) {
       port.close();
     }
   }
 
   String get path() { return _path; }
 
-  bool _exists(String path) native "Directory_Exists";
+  int _exists(String path) native "Directory_Exists";
   bool _create(String path) native "Directory_Create";
   bool _delete(String path) native "Directory_Delete";
 
   var _dirHandler;
   var _fileHandler;
   var _doneHandler;
   var _errorHandler;
 
   String _path;
 }