Make Transform a little more pleasant to use:

pkg/barback/test/transformer/one_to_many.dart

Index: pkg/barback/test/transformer/one_to_many.dart
diff --git a/pkg/barback/test/transformer/one_to_many.dart b/pkg/barback/test/transformer/one_to_many.dart
index e9d301d61907f56075925a68d8ba2dd075fce1f2..b3b2f5ee43374c46c1d2eefb299490f9d1dc02eb 100644
--- a/pkg/barback/test/transformer/one_to_many.dart
+++ b/pkg/barback/test/transformer/one_to_many.dart
@@ -29,7 +29,7 @@ class OneToManyTransformer extends MockTransformer {
         .then((input) => input.readAsString())
         .then((lines) {
       for (var line in lines.split(",")) {
-        var id = new AssetId(transform.primaryId.package, line);
+        var id = new AssetId(transform.primaryInput.id.package, line);
         transform.addOutput(new Asset.fromString(id, "spread $extension"));
       }
     });