Remove barback from the repo.

pkg/barback/test/transformer/lazy_aggregate_many_to_one.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_one;
-
-import 'package:barback/barback.dart';
-
-import 'declaring_aggregate_many_to_one.dart';
-
-/// Like [AggregateManyToOneTransformer], but returns a lazy asset that doesn't
-/// perform the rewrite until it's materialized.
-class LazyAggregateManyToOneTransformer
-    extends DeclaringAggregateManyToOneTransformer
-    implements LazyAggregateTransformer {
-  LazyAggregateManyToOneTransformer(String extension, String output)
-      : super(extension, output);
-}