Only pass an AssetId to isPrimary and declareOutputs.

pkg/barback/lib/src/declaring_transform.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library barback.declaring_transform;
 
 import 'asset_id.dart';
 import 'base_transform.dart';
 import 'transform_node.dart';
 
 /// A transform for [DeclaringTransform]ers that allows them to declare the ids
 /// of the outputs they'll generate without generating the concrete bodies of
 /// those outputs.
 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.
   ///
   /// A transformation can declare as many assets as it wants. If
   /// [DeclaringTransformer.declareOutputs] declareds a given asset id for a
   /// given input, [Transformer.apply] should emit the corresponding asset as
   /// well.
   void declareOutput(AssetId id) {
     // TODO(nweiz): This should immediately throw if an output with that ID
     // has already been declared by this transformer.
     _outputIds.add(id);
   }
 }
 
 /// The controller for [DeclaringTransform].
 class DeclaringTransformController extends BaseTransformController {
   DeclaringTransform get transform => super.transform;
 
   /// The set of ids that the transformer declares it will emit for the given
   /// primary input.
   Set<AssetId> get outputIds => transform._outputIds;
 
   DeclaringTransformController(TransformNode node)
       : super(new DeclaringTransform._(node));
 }