Only pass an AssetId to isPrimary and declareOutputs.

pkg/code_transformers/test/assets_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.assets_test;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:code_transformers/assets.dart';
@@ skipping 82 matching lines @@
         errorOnAbsolute: false,
         result: null);
   });
 }
 
 class Validator extends Transformer {
   final Function validation;
 
   Validator(this.validation);
 
-  Future<bool> isPrimary(Asset input) => new Future.value(true);
-
   Future apply(Transform transform) {
     return new Future.value(validation(transform));
   }
 }