Make Transform a little more pleasant to use:

pkg/barback/test/transformer/check_content.dart

Index: pkg/barback/test/transformer/check_content.dart
diff --git a/pkg/barback/test/transformer/check_content.dart b/pkg/barback/test/transformer/check_content.dart
index 445e86f529b37047aad447d5b30883aa20c2b408..c1706703c5444759eacb23c6526924f7b285a3d9 100644
--- a/pkg/barback/test/transformer/check_content.dart
+++ b/pkg/barback/test/transformer/check_content.dart
@@ -21,11 +21,9 @@ class CheckContentTransformer extends MockTransformer {
     asset.readAsString().then((value) => value.contains(content));
 
   Future doApply(Transform transform) {
-    return getPrimary(transform).then((primary) {
-      return primary.readAsString().then((value) {
-        transform.addOutput(
-            new Asset.fromString(primary.id, "$value$addition"));
-      });
+    return transform.primaryInput.readAsString().then((value) {
+      transform.addOutput(
+          new Asset.fromString(transform.primaryInput.id, "$value$addition"));
     });
   }
 }