Roll Observatory packages and add a roll script

packages/watcher/lib/src/async_queue.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.async_queue;
 
 import 'dart:async';
 import 'dart:collection';
 
 typedef Future ItemProcessor<T>(T item);
@@ skipping 53 matching lines @@
     var item = _items.removeFirst();
     return _processor(item).then((_) {
       if (_items.isNotEmpty) return _processNextItem();
 
       // We have drained the queue, stop processing and wait until something
       // has been enqueued.
       _isProcessing = false;
     });
   }
 }