Only pass an AssetId to isPrimary and declareOutputs.

pkg/barback/lib/src/declaring_transform.dart

Index: pkg/barback/lib/src/declaring_transform.dart
diff --git a/pkg/barback/lib/src/declaring_transform.dart b/pkg/barback/lib/src/declaring_transform.dart
index 3f6ecbd4fbd1a9ccafc8bd242cfe87501abec4c0..2f8a8bfda3db4093114b9deaa984fc7886ece9a6 100644
--- a/pkg/barback/lib/src/declaring_transform.dart
+++ b/pkg/barback/lib/src/declaring_transform.dart
@@ -14,8 +14,11 @@ import 'transform_node.dart';
 class DeclaringTransform extends BaseTransform {
   final _outputIds = new Set<AssetId>();
 
+  final AssetId primaryId;
+
   DeclaringTransform._(TransformNode node)
-    : super(node);
+      : primaryId = node.primary.id,
+        super(node);
 
   /// Stores [id] as the id of an output that will be created by this
   /// transformation when it's run.