Add a [Transform.hasInput] convenience method to Barback.

pkg/barback/test/utils.dart

Index: pkg/barback/test/utils.dart
diff --git a/pkg/barback/test/utils.dart b/pkg/barback/test/utils.dart
index 4d72a2b36061d49a7ad6ff077f7d8479dd475a8b..7a77f1bddd27dcbf89415413a13f7e5e5c2c8c8e 100644
--- a/pkg/barback/test/utils.dart
+++ b/pkg/barback/test/utils.dart
@@ -24,6 +24,7 @@ export 'transformer/check_content_and_rename.dart';
 export 'transformer/conditionally_consume_primary.dart';
 export 'transformer/create_asset.dart';
 export 'transformer/emit_nothing.dart';
+export 'transformer/has_input.dart';
 export 'transformer/lazy_bad.dart';
 export 'transformer/lazy_many_to_one.dart';
 export 'transformer/lazy_rewrite.dart';