Run pub tests against older versions of barback.

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

Index: third_party/pkg/barback-0.13.0/lib/src/errors.dart
diff --git a/pkg/barback/lib/src/errors.dart b/third_party/pkg/barback-0.13.0/lib/src/errors.dart
similarity index 93%
copy from pkg/barback/lib/src/errors.dart
copy to third_party/pkg/barback-0.13.0/lib/src/errors.dart
index 0c2b5c5dd5e3eb33eceb424152b6665406607350..5303b9d5629fc041bd6cea0d6675912d47567f8b 100644
--- a/pkg/barback/lib/src/errors.dart
+++ b/third_party/pkg/barback-0.13.0/lib/src/errors.dart
@@ -6,8 +6,8 @@ library barback.errors;
 
 import 'package:stack_trace/stack_trace.dart';
 
-import 'asset/asset_id.dart';
-import 'transformer/wrapping_aggregate_transformer.dart';
+import 'asset_id.dart';
+import 'transformer.dart';
 import 'utils.dart';
 
 /// Error thrown when an asset with [id] cannot be found.
@@ -167,17 +167,12 @@ class AssetLoadException extends _WrappedException {
 /// the transformer that is applied to it.
 class TransformInfo {
   /// The transformer that's run for this transform.
-  ///
-  /// This may be a [Transformer] or a [WrappingAggregateTransformer]. It may
-  /// also return additional types in the future.
-  final transformer;
+  final Transformer transformer;
 
   /// The id of this transform's primary asset.
   final AssetId primaryId;
 
-  TransformInfo(transformer, this.primaryId)
-      : transformer = transformer is WrappingAggregateTransformer ?
-            transformer.transformer : transformer;
+  TransformInfo(this.transformer, this.primaryId);
 
   bool operator==(other) =>
       other is TransformInfo &&