Remove barback from the repo.

pkg/barback/test/transformer/lazy_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_many_to_one;
-
-import 'package:barback/barback.dart';
-
-import 'many_to_one.dart';
-
-/// Like [ManyToOneTransformer], but returns a lazy asset that doesn't perform
-/// the conglomeration until it's materialized.
-class LazyManyToOneTransformer extends ManyToOneTransformer
-    implements LazyTransformer {
-  LazyManyToOneTransformer(String extension)
-      : super(extension);
-
-  void declareOutputs(DeclaringTransform transform) {
-    transform.declareOutput(transform.primaryId.changeExtension(".out"));
-  }
-}