Add FileSystemWatcher class to dart:io.

sdk/lib/io/file_impl.dart

 // Copyright (c) 2013, 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;
 
 // Read the file in blocks of size 64k.
 const int _BLOCK_SIZE = 64 * 1024;
 
 
@@ skipping 216 matching lines @@
 const int _STAT_REQUEST = 25;
 
 // TODO(ager): The only reason for this class is that the patching
 // mechanism doesn't seem to like patching a private top level
 // function.
 class _FileUtils {
   external static SendPort _newServicePort();
 }
 
 // Class for encapsulating the native implementation of files.
-class _File implements File {
+class _File extends FileSystemEntity implements File {
   final String path;
   SendPort _fileService;
 
   // Constructor for file.
   _File(String this.path) {
     if (path is! String) {
       throw new ArgumentError('${Error.safeToString(path)} '
                               'is not a String');
     }
   }
@@ skipping 729 matching lines @@
   void _checkNotClosed() {
     if (closed) {
       throw new FileException("File closed", path);
     }
   }
 
   Future _closedException() {
     return new Future.error(new FileException("File closed", path));
   }
 }