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..363d4d114c2d5056934ccb58eaebe6c89452ee5d 100644
--- a/pkg/barback/test/transformer/rewrite.dart
+++ b/pkg/barback/test/transformer/rewrite.dart
@@ -29,14 +29,12 @@ class RewriteTransformer extends MockTransformer {
     new Future.value(asset.id.extension == ".$from");
 
   Future doApply(Transform transform) {
-    return getPrimary(transform).then((input) {
-      return Future.wait(to.split(" ").map((extension) {
-        var id = transform.primaryId.changeExtension(".$extension");
-        return input.readAsString().then((content) {
-          transform.addOutput(new Asset.fromString(id, "$content.$extension"));
-        });
-      }));
-    });
+    return Future.wait(to.split(" ").map((extension) {
+      var id = transform.primaryInput.id.changeExtension(".$extension");
+      return transform.primaryInput.readAsString().then((content) {
+        transform.addOutput(new Asset.fromString(id, "$content.$extension"));
+      });
+    }));
   }
 
   String toString() => "$from->$to";