Run pub tests against older versions of barback.

third_party/pkg/barback-0.13.0/test/transformer/create_asset.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.create_asset;
 
+import 'dart:async';
+
 import 'package:barback/barback.dart';
+import 'package:barback/src/utils.dart';
 
 import 'mock.dart';
 
 /// A transformer that outputs an asset with the given id.
 class CreateAssetTransformer extends MockTransformer {
   final String output;
 
   CreateAssetTransformer(this.output);
 
-  bool doIsPrimary(AssetId id) => true;
+  Future<bool> doIsPrimary(_) => new Future.value(true);
 
-  void doApply(Transform transform) {
-    transform.addOutput(
-        new Asset.fromString(new AssetId.parse(output), output));
+  Future doApply(Transform transform) {
+    return newFuture(() {
+      transform.addOutput(
+          new Asset.fromString(new AssetId.parse(output), output));
+    });
   }
 }