Run pub tests against older versions of barback.

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

Index: third_party/pkg/barback-0.13.0/lib/src/declaring_transformer.dart
diff --git a/pkg/barback/lib/src/transformer/declaring_transformer.dart b/third_party/pkg/barback-0.13.0/lib/src/declaring_transformer.dart
similarity index 82%
copy from pkg/barback/lib/src/transformer/declaring_transformer.dart
copy to third_party/pkg/barback-0.13.0/lib/src/declaring_transformer.dart
index d9e5adcc54d61fc8d56b7b4998583344243437b7..a784493da9cf3268665ff14b545e6a433379da28 100644
--- a/pkg/barback/lib/src/transformer/declaring_transformer.dart
+++ b/third_party/pkg/barback-0.13.0/lib/src/declaring_transformer.dart
@@ -2,7 +2,7 @@
 // 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.declaring_transformer;
+library barback.declaring_transformer;
 
 import 'dart:async';
 
@@ -21,8 +21,5 @@ abstract class DeclaringTransformer {
   /// First, instead of having access to the primary input's contents, it only
   /// has access to its id. Second, instead of emitting [Asset]s, it just emits
   /// [AssetId]s through [transform.addOutputId].
-  ///
-  /// If this does asynchronous work, it should return a [Future] that completes
-  /// once it's finished.
-  declareOutputs(DeclaringTransform transform);
+  Future declareOutputs(DeclaringTransform transform);
 }