Only run [declareOutputs] once for each asset/transformer pair.

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 7a77f1bddd27dcbf89415413a13f7e5e5c2c8c8e..66c6abfbb77f682c8c4494b0c0681689ba1823a0 100644
--- a/pkg/barback/test/utils.dart
+++ b/pkg/barback/test/utils.dart
@@ -23,6 +23,9 @@ export 'transformer/check_content.dart';
 export 'transformer/check_content_and_rename.dart';
 export 'transformer/conditionally_consume_primary.dart';
 export 'transformer/create_asset.dart';
+export 'transformer/declare_assets.dart';
+export 'transformer/declaring_bad.dart';
+export 'transformer/declaring_rewrite.dart';
 export 'transformer/emit_nothing.dart';
 export 'transformer/has_input.dart';
 export 'transformer/lazy_bad.dart';