Only pass an AssetId to isPrimary and declareOutputs.

pkg/barback/test/transformer/check_content_and_rename.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_and_rename;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 
 import 'mock.dart';
 
 /// A transformer that checks the extension and content of an asset, then
 /// produces a new asset with a new extension and new content.
 class CheckContentAndRenameTransformer extends MockTransformer {
   final String oldExtension;
   final String oldContent;
   final String newExtension;
   final String newContent;
 
   CheckContentAndRenameTransformer(this.oldExtension, this.oldContent,
       this.newExtension, this.newContent);
 
-  Future<bool> doIsPrimary(Asset asset) {
-    if (asset.id.extension != '.$oldExtension') return new Future.value(false);
-    return asset.readAsString().then((value) => value == oldContent);
-  }
+  Future<bool> doIsPrimary(AssetId id) =>
+      new Future.value(id.extension != '.$oldExtension');
 
   Future doApply(Transform transform) {
     return getPrimary(transform).then((input) {
-      transform.addOutput(new Asset.fromString(
-          input.id.changeExtension('.$newExtension'), newContent));
+      return input.readAsString().then((contents) {
+        if (contents != oldContent) return;
+
+        transform.addOutput(new Asset.fromString(
+            input.id.changeExtension('.$newExtension'), newContent));
+      });
     });
   }
 }