Remove barback from the repo.

pkg/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.
-///
-/// If a transformer implements this interface, that allows barback to perform
-/// optimizations to make the asset graph work more smoothly.
-abstract class DeclaringAggregateTransformer {
-  /// Declare which assets would be emitted for the primary input ids specified
-  /// by [transform].
-  ///
-  /// This works a little like [AggregateTransformer.apply], with two main
-  /// differences. First, instead of having access to the primary inputs'
-  /// contents, it only has access to their ids. Second, instead of emitting
-  /// [Asset]s, it just emits [AssetId]s through [transform.addOutputId].
-  ///
-  /// 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);
-}