Fix type error in implementation of dart:io Directory.existsSync.

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 extends FileSystemEntity implements Directory {
   final String path;
 
   _Directory(this.path) {
     if (path is! String) {
       throw new ArgumentError('${Error.safeToString(path)} '
                               'is not a String');
     }
   }
 
   external static _current();
   external static _setCurrent(path);
   external static _createTemp(String path);
   external static String _systemTemp();
-  external static int _exists(String path);
+  external static _exists(String path);
   external static _create(String path);
   external static _deleteNative(String path, bool recursive);
   external static _rename(String path, String newPath);
   external static List _list(String path, bool recursive, bool followLinks);
 
   static Directory get current {
     var result = _current();
     if (result is OSError) {
       throw new FileSystemException(
           "Getting current working directory failed", "", result);
@@ skipping 345 matching lines @@
       controller.addError(
           new FileSystemException("Directory listing failed",
                                  errorPath,
                                  err));
     } else {
       controller.addError(
           new FileSystemException("Internal error"));
     }
   }
 }