Roll Observatory packages and add a roll script

packages/watcher/lib/src/directory_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.directory_watcher;
 
 import 'dart:io';
 
 import 'watch_event.dart';
 import '../watcher.dart';
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   /// watchers.
   factory DirectoryWatcher(String directory, {Duration pollingDelay}) {
     if (FileSystemEntity.isWatchSupported) {
       if (Platform.isLinux) return new LinuxDirectoryWatcher(directory);
       if (Platform.isMacOS) return new MacOSDirectoryWatcher(directory);
       if (Platform.isWindows) return new WindowsDirectoryWatcher(directory);
     }
     return new PollingDirectoryWatcher(directory, pollingDelay: pollingDelay);
   }
 }