Roll Observatory packages and add a roll script

packages/watcher/lib/watcher.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;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'src/watch_event.dart';
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   /// and higher CPU usage. Defaults to one second. Ignored for non-polling
   /// watchers.
   factory Watcher(String path, {Duration pollingDelay}) {
     if (new File(path).existsSync()) {
       return new FileWatcher(path, pollingDelay: pollingDelay);
     } else {
       return new DirectoryWatcher(path, pollingDelay: pollingDelay);
     }
   }
 }