Roll Observatory packages and add a roll script

packages/barback/lib/src/graph/package_graph.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.package_graph;
 
 import 'dart:async';
 import 'dart:collection';
 
 import '../asset/asset_id.dart';
@@ skipping 271 matching lines @@
         completer.completeError(error, stackTrace);
       });
     }, onError: (error, stackTrace) {
       _lastUnexpectedError = error;
       _lastUnexpectedErrorTrace = stackTrace;
       _resultsController.addError(error, stackTrace);
     });
     return completer.future;
   }
 }