Run pub tests against older versions of barback.

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

Index: third_party/pkg/barback-0.13.0/test/transformer/lazy_many_to_one.dart
diff --git a/pkg/barback/test/transformer/lazy_many_to_one.dart b/third_party/pkg/barback-0.13.0/test/transformer/lazy_many_to_one.dart
similarity index 90%
copy from pkg/barback/test/transformer/lazy_many_to_one.dart
copy to third_party/pkg/barback-0.13.0/test/transformer/lazy_many_to_one.dart
index ef407ef0369f4d1a3e52542a6f92f706f823396f..dcdc0b16b31c0743931b238d3dab161eab266ed5 100644
--- a/pkg/barback/test/transformer/lazy_many_to_one.dart
+++ b/third_party/pkg/barback-0.13.0/test/transformer/lazy_many_to_one.dart
@@ -4,6 +4,8 @@
 
 library barback.test.transformer.lazy_many_to_one;
 
+import 'dart:async';
+
 import 'package:barback/barback.dart';
 
 import 'many_to_one.dart';
@@ -15,7 +17,7 @@ class LazyManyToOneTransformer extends ManyToOneTransformer
   LazyManyToOneTransformer(String extension)
       : super(extension);
 
-  void declareOutputs(DeclaringTransform transform) {
+  Future declareOutputs(DeclaringTransform transform) {
     transform.declareOutput(transform.primaryId.changeExtension(".out"));
   }
 }