add file watching support

pkg/analyzer/lib/file_system/file_system.dart

Index: pkg/analyzer/lib/file_system/file_system.dart
diff --git a/pkg/analyzer/lib/file_system/file_system.dart b/pkg/analyzer/lib/file_system/file_system.dart
index 6fb98850cffcabecbf0e15b1470eca9a7a072093..9a41f8129d2a47ee9acc7b583a1254077d17e270 100644
--- a/pkg/analyzer/lib/file_system/file_system.dart
+++ b/pkg/analyzer/lib/file_system/file_system.dart
@@ -15,6 +15,11 @@ import 'package:watcher/watcher.dart';
  */
 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.
    */