Only pass an AssetId to isPrimary and declareOutputs.

sdk/lib/_internal/pub/asset/dart/serialize/transformer.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 pub.asset.serialize.transformer;
 
 import 'dart:isolate';
 
 import 'package:barback/barback.dart';
 // TODO(nweiz): don't import from "src" once issue 14966 is fixed.
 import 'package:barback/src/internal_asset.dart';
 
 import '../serialize.dart';
 import 'transform.dart';
 
 /// Converts [transformer] into a serializable map.
 Map serializeTransformer(Transformer transformer) {
   var port = new ReceivePort();
   port.listen((wrappedMessage) {
     respond(wrappedMessage, (message) {
       if (message['type'] == 'isPrimary') {
-        return transformer.isPrimary(deserializeAsset(message['asset']));
+        return transformer.isPrimary(deserializeId(message['id']));
       } else {
         assert(message['type'] == 'apply');
 
         // Make sure we return null so that if the transformer's [apply] returns
         // a non-serializable value it doesn't cause problems.
         return transformer.apply(
             new ForeignTransform(message['transform'])).then((_) => null);
       }
     });
   });
@@ skipping 18 matching lines @@
 
 /// Converts [transformerOrGroup] into a serializable map.
 Map serializeTransformerOrGroup(transformerOrGroup) {
   if (transformerOrGroup is Transformer) {
     return serializeTransformer(transformerOrGroup);
   } else {
     assert(transformerOrGroup is TransformerGroup);
     return serializeTransformerGroup(transformerOrGroup);
   }
 }