Roll Observatory packages and add a roll script

packages/watcher/lib/src/constructable_file_system_event.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.
 
 library watcher.constructable_file_system_event;
 
 import 'dart:io';
 
 abstract class _ConstructableFileSystemEvent implements FileSystemEvent {
   final bool isDirectory;
@@ skipping 40 matching lines @@
     implements FileSystemMoveEvent {
   final String destination;
   final type = FileSystemEvent.MOVE;
 
   ConstructableFileSystemMoveEvent(String path, bool isDirectory,
       this.destination)
       : super(path, isDirectory);
 
   String toString() => "FileSystemMoveEvent('$path', '$destination')";
 }