Make Transform a little more pleasant to use:

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 4d3faa883e0d76dfe5c6093c2fa718de997dc2da..6525b07ad64b0b2afea2b129e6428295f88e2cab 100644
--- a/pkg/barback/test/transformer/rewrite.dart
+++ b/pkg/barback/test/transformer/rewrite.dart
@@ -31,7 +31,7 @@ class RewriteTransformer extends MockTransformer {
   Future doApply(Transform transform) {
     return getPrimary(transform).then((input) {
       return Future.wait(to.split(" ").map((extension) {
-        var id = transform.primaryId.changeExtension(".$extension");
+        var id = input.id.changeExtension(".$extension");
         return input.readAsString().then((content) {
           transform.addOutput(new Asset.fromString(id, "$content.$extension"));
         });