Only pass an AssetId to isPrimary and declareOutputs.

pkg/barback/test/transformer/catch_asset_not_found.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 barback.test.transformer.catch_asset_not_found;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:barback/src/utils.dart';
 
 import 'mock.dart';
 
 /// A transformer that tries to load a secondary input and catches an
 /// [AssetNotFoundException] if the input doesn't exist.
 class CatchAssetNotFoundTransformer extends MockTransformer {
   /// The extension of assets this applies to.
   final String extension;
 
   /// The id of the secondary input to load.
   final AssetId input;
 
   CatchAssetNotFoundTransformer(this.extension, String input)
       : input = new AssetId.parse(input);
 
-  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) {
     return transform.getInput(input).then((_) {
       transform.addOutput(new Asset.fromString(
           transform.primaryInput.id, "success"));
     }).catchError((e) {
       if (e is! AssetNotFoundException) throw e;
       transform.addOutput(new Asset.fromString(
           transform.primaryInput.id, "failed to load $input"));
     });
   }
 }