Remove barback from the repo.

pkg/barback/test/transformer/aggregate_many_to_many.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.test.transformer.aggregate_many_to_many;
-
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-import 'package:path/path.dart' as path;
-
-import 'mock_aggregate.dart';
-
-/// An [AggregateTransformer] that takes all assets with a given extension,
-/// grouped by directory, adds to their contents.
-class AggregateManyToManyTransformer extends MockAggregateTransformer {
-  /// The extension of assets to combine.
-  final String extension;
-
-  AggregateManyToManyTransformer(this.extension);
-
-  String doClassifyPrimary(AssetId id) {
-    if (id.extension != ".$extension") return null;
-    return path.url.dirname(id.path);
-  }
-
-  Future doApply(AggregateTransform transform) {
-    return getPrimaryInputs(transform).asyncMap((asset) {
-      return asset.readAsString().then((contents) {
-        transform.addOutput(new Asset.fromString(
-            asset.id, "modified $contents"));
-      });
-    }).toList();
-  }
-
-  String toString() => "aggregate $extension->many";
-}