Only pass an AssetId to isPrimary and declareOutputs.

pkg/barback/test/transformer/check_content.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.check_content;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 
 import 'mock.dart';
 
 /// A transformer that modifies assets that contains the given content.
 class CheckContentTransformer extends MockTransformer {
   final Pattern content;
   final String addition;
 
   CheckContentTransformer(this.content, this.addition);
 
-  Future<bool> doIsPrimary(Asset asset) =>
-    asset.readAsString().then((value) => value.contains(content));
+  Future<bool> doIsPrimary(AssetId id) => new Future.value(true);
 
   Future doApply(Transform transform) {
     return getPrimary(transform).then((primary) {
       return primary.readAsString().then((value) {
+        if (!value.contains(content)) return;
+
         transform.addOutput(
             new Asset.fromString(primary.id, "$value$addition"));
       });
     });
   }
 }