Roll Observatory packages and add a roll script

packages/barback/lib/src/transformer/declaring_aggregate_transformer.dart

 // Copyright (c) 2014, 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.transformer.declaring_aggregate_transformer;
 
 import 'declaring_aggregate_transform.dart';
 
 /// An interface for [Transformer]s that can cheaply figure out which assets
 /// they'll emit without doing the work of actually creating those assets.
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   ///
   /// If this does asynchronous work, it should return a [Future] that completes
   /// once it's finished.
   ///
   /// This may complete before [DeclaringAggregateTransform.primaryIds] stream
   /// is closed. For example, it may know that each key will only have two
   /// inputs associated with it, and so use `transform.primaryIds.take(2)` to
   /// access only those inputs' ids.
   declareOutputs(DeclaringAggregateTransform transform);
 }