Only pass an AssetId to isPrimary and declareOutputs.

pkg/barback/test/transformer/rewrite.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.rewrite;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 
 import 'mock.dart';
 
 /// A [Transformer] that takes assets ending with one extension and generates
 /// assets with a given extension.
 ///
 /// Appends the output extension to the contents of the input file.
 class RewriteTransformer extends MockTransformer {
   final String from;
   final String to;
 
   /// Creates a transformer that rewrites assets whose extension is [from] to
   /// one whose extension is [to].
   ///
   /// [to] may be a space-separated list in which case multiple outputs will be
   /// created for each input.
   RewriteTransformer(this.from, this.to);
 
-  Future<bool> doIsPrimary(Asset asset) =>
-    new Future.value(asset.id.extension == ".$from");
+  Future<bool> doIsPrimary(AssetId id) =>
+      new Future.value(id.extension == ".$from");
 
   Future doApply(Transform transform) {
     return getPrimary(transform).then((input) {
       return Future.wait(to.split(" ").map((extension) {
         var id = input.id.changeExtension(".$extension");
         return input.readAsString().then((content) {
           transform.addOutput(new Asset.fromString(id, "$content.$extension"));
         });
       }));
     });
   }
 
   String toString() => "$from->$to";
 }