Roll Observatory packages and add a roll script

packages/watcher/lib/src/file_watcher.dart

 // Copyright (c) 2015, 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.file_watcher;
 
 import 'dart:io';
 
 import 'watch_event.dart';
 import '../watcher.dart';
@@ skipping 23 matching lines @@
   factory FileWatcher(String file, {Duration pollingDelay}) {
     // [File.watch] doesn't work on Windows, but
     // [FileSystemEntity.isWatchSupported] is still true because directory
     // watching does work.
     if (FileSystemEntity.isWatchSupported && !Platform.isWindows) {
       return new NativeFileWatcher(file);
     }
     return new PollingFileWatcher(file, pollingDelay: pollingDelay);
   }
 }