Only pass an AssetId to isPrimary and declareOutputs.

pkg/barback/test/transformer/has_input.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.has_input;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 
 import 'mock.dart';
 
 /// Overwrites its primary inputs with descriptions of whether various secondary
 /// inputs exist.
 class HasInputTransformer extends MockTransformer {
   /// The inputs whose existence will be checked.
   final List<AssetId> inputs;
 
   HasInputTransformer(Iterable<String> inputs)
       : inputs = inputs.map((input) => new AssetId.parse(input)).toList();
 
-  Future<bool> doIsPrimary(Asset asset) => new Future.value(true);
+  Future<bool> doIsPrimary(_) => new Future.value(true);
 
   Future doApply(Transform transform) {
     return Future.wait(inputs.map((input) {
       return transform.hasInput(input).then((hasInput) => "$input: $hasInput");
     })).then((results) {
       transform.addOutput(new Asset.fromString(
           transform.primaryInput.id, results.join(', ')));
     });
   }
 
   String toString() => "has inputs $inputs";
 }