Roll Observatory packages and add a roll script

packages/barback/lib/src/graph/asset_cascade.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 barback.graph.asset_cascade;
 
 import 'dart:async';
 
 import '../asset/asset.dart';
 import '../asset/asset_id.dart';
@@ skipping 208 matching lines @@
     _streams.onLogPool.add(phase.onLog);
     if (_phaseStatusSubscription != null) _phaseStatusSubscription.cancel();
     _phaseStatusSubscription =
         phase.onStatusChange.listen(_streams.changeStatus);
 
     _phases.add(phase);
   }
 
   String toString() => "cascade for $package";
 }