Remove barback from the repo.

pkg/barback/test/transformer/declaring_aggregate_many_to_many.dart

Index: pkg/barback/test/transformer/declaring_aggregate_many_to_many.dart
diff --git a/pkg/barback/test/transformer/declaring_aggregate_many_to_many.dart b/pkg/barback/test/transformer/declaring_aggregate_many_to_many.dart
deleted file mode 100644
index 237734514650ae6ed210f70d1a6bd224145e0f61..0000000000000000000000000000000000000000
--- a/pkg/barback/test/transformer/declaring_aggregate_many_to_many.dart
+++ /dev/null
@@ -1,23 +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.test.transformer.declaring_aggregate_many_to_many;
-
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-import 'aggregate_many_to_many.dart';
-
-/// Like [AggregateManyToManyTransformer], but declares its assets ahead of
-/// time.
-class DeclaringAggregateManyToManyTransformer
-    extends AggregateManyToManyTransformer
-    implements DeclaringAggregateTransformer {
-  DeclaringAggregateManyToManyTransformer(String extension)
-      : super(extension);
-
-  Future declareOutputs(DeclaringAggregateTransform transform) =>
-      transform.primaryIds.asyncMap(transform.declareOutput).toList();
-}