Only pass an AssetId to isPrimary and declareOutputs.

pkg/barback/test/transformer/lazy_rewrite.dart

Index: pkg/barback/test/transformer/lazy_rewrite.dart
diff --git a/pkg/barback/test/transformer/lazy_rewrite.dart b/pkg/barback/test/transformer/lazy_rewrite.dart
index f694a0e77da6fd95176075593ed1be8179661b34..1a3677877684151bb3e210fe6c190b7030981956 100644
--- a/pkg/barback/test/transformer/lazy_rewrite.dart
+++ b/pkg/barback/test/transformer/lazy_rewrite.dart
@@ -20,7 +20,7 @@ class LazyRewriteTransformer extends RewriteTransformer
   Future declareOutputs(DeclaringTransform transform) {
     if (consumePrimary) transform.consumePrimary();
     for (var extension in to.split(" ")) {
-      var id = transform.primaryInput.id.changeExtension(".$extension");
+      var id = transform.primaryId.changeExtension(".$extension");
       transform.declareOutput(id);
     }
   }