Remove barback from the repo.

pkg/barback/lib/src/transformer/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.aggregate_transformer;
-
-import '../asset/asset_id.dart';
-import 'aggregate_transform.dart';
-
-/// An alternate interface for transformers that want to perform aggregate
-/// transformations on multiple inputs without any individual one of them being
-/// considered "primary".
-///
-/// This is useful for transformers like image spriting, where all the images in
-/// a directory need to be combined into a single image. A normal [Transformer]
-/// can't do this gracefully since when it's running on a single image, it has
-/// no way of knowing what other images exist to request as secondary inputs.
-///
-/// Aggregate transformers work by classifying assets into different groups
-/// based on their ids in [classifyPrimary]. Then [apply] is run once for each
-/// group. For example, a spriting transformer might put each image asset into a
-/// group identified by its directory name. All images in a given directory will
-/// end up in the same group, and they'll all be passed to one [apply] call.
-///
-/// If possible, aggregate transformers should implement
-/// [DeclaringAggregateTransformer] as well to help barback optimize the package
-/// graph.
-abstract class AggregateTransformer {
-  /// Classifies an asset id by returning a key identifying which group the
-  /// asset should be placed in.
-  ///
-  /// All assets for which [classifyPrimary] returns the same key are passed
-  /// together to the same [apply] call.
-  ///
-  /// This may return [Future<String>] or, if it's entirely synchronous,
-  /// [String]. Any string can be used to classify an asset. If possible,
-  /// though, this should return a path-like string to aid in logging.
-  ///
-  /// A return value of `null` indicates that the transformer is not interested
-  /// in an asset. Assets with a key of `null` will not be passed to any [apply]
-  /// call; this is equivalent to [Transformer.isPrimary] returning `false`.
-  classifyPrimary(AssetId id);
-
-  /// Runs this transformer on a group of primary inputs specified by
-  /// [transform].
-  ///
-  /// If this does asynchronous work, it should return a [Future] that completes
-  /// once it's finished.
-  ///
-  /// This may complete before [AggregateTransform.primarInputs] is closed. For
-  /// example, it may know that each key will only have two inputs associated
-  /// with it, and so use `transform.primaryInputs.take(2)` to access only those
-  /// inputs.
-  apply(AggregateTransform transform);
-
-  String toString() => runtimeType.toString().replaceAll("Transformer", "");
-}