Remove barback from the repo.

pkg/barback/lib/src/transformer/lazy_aggregate_transformer.dart

Index: pkg/barback/lib/src/transformer/lazy_aggregate_transformer.dart
diff --git a/pkg/barback/lib/src/transformer/lazy_aggregate_transformer.dart b/pkg/barback/lib/src/transformer/lazy_aggregate_transformer.dart
deleted file mode 100644
index 891fc1ec29fbf361a7aff0f6b9dfb6247bfb3a3c..0000000000000000000000000000000000000000
--- a/pkg/barback/lib/src/transformer/lazy_aggregate_transformer.dart
+++ /dev/null
@@ -1,16 +0,0 @@
-// 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.lazy_aggregate_transformer;
-
-import 'declaring_aggregate_transformer.dart';
-
-/// An interface for [AggregateTransformer]s that indicates that the
-/// transformer's outputs shouldn't be generated until requested.
-///
-/// The [declareOutputs] method is used to figure out which assets should be
-/// treated as "lazy." Lazy assets will only be forced to be generated if
-/// they're requested by the user or if they're used by a non-declaring
-/// transformer.
-abstract class LazyAggregateTransformer extends DeclaringAggregateTransformer {}