Only pass an AssetId to isPrimary and declareOutputs.

pkg/barback/test/transformer/mock.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.mock;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:barback/src/utils.dart';
@@ skipping 148 matching lines @@
 
   /// Like [Transform.primaryInput], but respects [pauseGetPrimary].
   ///
   /// This is intended for use by subclasses of [MockTransformer].
   Future<Asset> getPrimary(Transform transform) {
     return newFuture(() {
       if (_primaryInput != null) return _primaryInput.future;
     }).then((_) => transform.primaryInput);
   }
 
-  Future<bool> isPrimary(Asset asset) {
-    return newFuture(() => doIsPrimary(asset)).then((result) {
+  Future<bool> isPrimary(AssetId id) {
+    return newFuture(() => doIsPrimary(id)).then((result) {
       return newFuture(() {
-        if (_isPrimary.containsKey(asset.id)) {
-          return _isPrimary[asset.id].future;
+        if (_isPrimary.containsKey(id)) {
+          return _isPrimary[id].future;
         }
       }).then((_) => result);
     });
   }
 
   Future apply(Transform transform) {
     _numRuns++;
     if (_runningTransforms == 0) _started.complete();
     _runningTransforms++;
     if (consumePrimary) transform.consumePrimary();
     return newFuture(() => doApply(transform)).then((_) {
       if (_apply != null) return _apply.future;
     }).whenComplete(() {
       _runningTransforms--;
       if (_runningTransforms == 0) _started = new Completer();
     });
   }
 
   /// The wrapped version of [isPrimary] for subclasses to override.
-  Future<bool> doIsPrimary(Asset asset);
+  Future<bool> doIsPrimary(AssetId id);
 
   /// The wrapped version of [doApply] for subclasses to override.
   Future doApply(Transform transform);
 }