Add FileSystemWatcher class to dart:io.

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 {
+class _Directory extends FileSystemEntity 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_START_REQUEST = 4;
   static const LIST_NEXT_REQUEST = 5;
   static const LIST_STOP_REQUEST = 6;
   static const RENAME_REQUEST = 7;
 
   final String path;
@@ skipping 394 matching lines @@
       controller.addError(
           new DirectoryException("Directory listing failed",
                                  errorPath,
                                  err));
     } else {
       controller.addError(
           new DirectoryException("Internal error"));
     }
   }
 }