Only pass an AssetId to isPrimary and declareOutputs.

pkg/code_transformers/test/resolver_test.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 code_transformers.test.resolver_test;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:code_transformers/resolver.dart';
@@ skipping 336 matching lines @@
 }
 
 class TestTransformer extends Transformer with ResolverTransformer {
   final AssetId primary;
   final Function validator;
 
   TestTransformer(Resolvers resolvers, this.primary, this.validator) {
     this.resolvers = resolvers;
   }
 
-  Future<bool> isPrimary(Asset input) =>
-      new Future.value(input.id == primary);
+  // TODO(nweiz): This should just take an AssetId when barback <0.13.0 support
+  // is dropped.
+  Future<bool> isPrimary(idOrAsset) {
+    var id = idOrAsset is AssetId ? idOrAsset : idOrAsset.id;
+    return new Future.value(id == primary);
+  }
 
   applyResolver(Transform transform, Resolver resolver) => validator(resolver);
 }