Refactor the barback tests somewhat.

pkg/barback/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.
+
+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);
+
+  Future<bool> doIsPrimary(Asset asset) => new Future.value(true);
+
+  Future doApply(Transform transform) {
+    return newFuture(() {
+      transform.addOutput(
+          new Asset.fromString(new AssetId.parse(output), output));
+    });
+  }
+}