Only pass an AssetId to isPrimary and declareOutputs.

sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart

 // Copyright (c) 2013, 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.load_transformers;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:isolate';
 
@@ skipping 73 matching lines @@
   /// transformer.
   final SendPort _port;
 
   /// The result of calling [toString] on the transformer in the isolate.
   final String _toString;
 
   _ForeignTransformer(Map map)
       : _port = map['port'],
         _toString = map['toString'];
 
-  Future<bool> isPrimary(Asset asset) {
+  Future<bool> isPrimary(AssetId id) {
     return call(_port, {
       'type': 'isPrimary',
-      'asset': serializeAsset(asset)
+      'id': serializeId(id)
     });
   }
 
   Future apply(Transform transform) {
     return call(_port, {
       'type': 'apply',
       'transform': serializeTransform(transform)
     });
   }
 
@@ skipping 20 matching lines @@
 /// Converts a serializable map into a [Transformer] or a [TransformerGroup].
 _deserializeTransformerOrGroup(Map map, TransformerId id) {
   if (map['type'] == 'Transformer') {
     var transformer = new _ForeignTransformer(map);
     return ExcludingTransformer.wrap(transformer, id.includes, id.excludes);
   }
 
   assert(map['type'] == 'TransformerGroup');
   return new _ForeignGroup(id, map);
 }