Remove barback from the repo.

pkg/barback/test/transformer/lazy_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.lazy_aggregate_many_to_many;
-
-import 'package:barback/barback.dart';
-
-import 'declaring_aggregate_many_to_many.dart';
-
-/// An [AggregateTransformer] that takes all assets in each directory with a
-/// given extension and adds to their contents.
-class LazyAggregateManyToManyTransformer
-    extends DeclaringAggregateManyToManyTransformer
-    implements LazyAggregateTransformer {
-  LazyAggregateManyToManyTransformer(String extension)
-      : super(extension);
-}