Run pub tests against older versions of barback.

third_party/pkg/barback-0.13.0/test/transformer/mock.dart

Index: third_party/pkg/barback-0.13.0/test/transformer/mock.dart
diff --git a/pkg/barback/test/transformer/mock.dart b/third_party/pkg/barback-0.13.0/test/transformer/mock.dart
similarity index 96%
copy from pkg/barback/test/transformer/mock.dart
copy to third_party/pkg/barback-0.13.0/test/transformer/mock.dart
index 8b0f5e296acfdfcb26b6466478581667d8dea24b..531f9e4dbeb98db4e33da09af412e8acdaab9699 100644
--- a/pkg/barback/test/transformer/mock.dart
+++ b/third_party/pkg/barback-0.13.0/test/transformer/mock.dart
@@ -190,14 +190,8 @@ abstract class MockTransformer extends Transformer {
   }
 
   /// The wrapped version of [isPrimary] for subclasses to override.
-  ///
-  /// This may return a `Future<bool>` or, if it's entirely synchronous, a
-  /// `bool`.
-  doIsPrimary(AssetId id);
+  Future<bool> doIsPrimary(AssetId id);
 
   /// The wrapped version of [doApply] for subclasses to override.
-  ///
-  /// If this does asynchronous work, it should return a [Future] that completes
-  /// once it's finished.
-  doApply(Transform transform);
+  Future doApply(Transform transform);
 }