add file watching support

pkg/analyzer/lib/file_system/file_system.dart

 // Copyright (c) 2014, 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.
 
 library file_system;
 
 import 'dart:async';
 
 import 'package:analyzer/src/generated/source.dart';
 import 'package:path/path.dart';
 import 'package:watcher/watcher.dart';
 
 /**
  * [File]s are leaf [Resource]s which contain data.
  */
 abstract class File extends Resource {
   /**
+   * Watch for changes to this file
+   */
+  Stream<WatchEvent> get changes;
+  
+  /**
    * Return the last-modified stamp of the file.
    * Throws [FileSystemException] if the file does not exist.
    */
   int get modificationStamp;
 
   /**
    * Create a new [Source] instance that serves this file.
    */
   Source createSource([Uri uri]);
 
@@ skipping 147 matching lines @@
   }
 
   @override
   Uri restoreAbsolute(Source source) => source.uri;
 
   /**
    * Return `true` if the given [uri] is a `file` URI.
    */
   static bool _isFileUri(Uri uri) => uri.scheme == _FILE_SCHEME;
 }