Only pass an AssetId to isPrimary and declareOutputs.

pkg/barback/test/transformer/rewrite.dart

Index: pkg/barback/test/transformer/rewrite.dart
diff --git a/pkg/barback/test/transformer/rewrite.dart b/pkg/barback/test/transformer/rewrite.dart
index 6525b07ad64b0b2afea2b129e6428295f88e2cab..995c94fff0c92cd474df4e4aa1cc3a71370de048 100644
--- a/pkg/barback/test/transformer/rewrite.dart
+++ b/pkg/barback/test/transformer/rewrite.dart
@@ -25,8 +25,8 @@ class RewriteTransformer extends MockTransformer {
   /// created for each input.
   RewriteTransformer(this.from, this.to);
 
-  Future<bool> doIsPrimary(Asset asset) =>
-    new Future.value(asset.id.extension == ".$from");
+  Future<bool> doIsPrimary(AssetId id) =>
+      new Future.value(id.extension == ".$from");
 
   Future doApply(Transform transform) {
     return getPrimary(transform).then((input) {