Run pub tests against older versions of barback.

third_party/pkg/barback-0.13.0/lib/barback.dart

Index: third_party/pkg/barback-0.13.0/lib/barback.dart
diff --git a/third_party/pkg/barback-0.13.0/lib/barback.dart b/third_party/pkg/barback-0.13.0/lib/barback.dart
new file mode 100644
index 0000000000000000000000000000000000000000..cbb55fcf8af9c8553b5638f438a101957defdcdc
--- /dev/null
+++ b/third_party/pkg/barback-0.13.0/lib/barback.dart
@@ -0,0 +1,23 @@
+// Copyright (c) 2013, 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;
+
+export 'src/asset.dart';
+export 'src/asset_id.dart';
+export 'src/asset_set.dart';
+export 'src/barback.dart';
+export 'src/barback_settings.dart';
+export 'src/base_transform.dart';
+export 'src/build_result.dart';
+export 'src/declaring_transform.dart' hide DeclaringTransformController;
+export 'src/declaring_transformer.dart';
+export 'src/errors.dart' hide flattenAggregateExceptions;
+export 'src/lazy_transformer.dart';
+export 'src/log.dart';
+export 'src/package_provider.dart';
+export 'src/transform.dart' hide TransformController;
+export 'src/transform_logger.dart';
+export 'src/transformer.dart';
+export 'src/transformer_group.dart';