Only pass an AssetId to isPrimary and declareOutputs.

pkg/barback/test/transformer/emit_nothing.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.test.transformer.emit_nothing;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 
 import 'mock.dart';
 
 /// A transformer that emits no assets.
 class EmitNothingTransformer extends MockTransformer {
   final String extension;
 
   EmitNothingTransformer(this.extension);
 
-  Future<bool> doIsPrimary(Asset asset) =>
-    new Future.value(asset.id.extension == ".$extension");
+  Future<bool> doIsPrimary(AssetId id) =>
+    new Future.value(id.extension == ".$extension");
 
   Future doApply(Transform transform) => new Future.value();
 
   String toString() => "$extension->nothing";
 }