Refactor the barback tests somewhat.

pkg/barback/test/transformer/check_content.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 'mock.dart';
+
+/// A transformer that modifies assets with the given content.
+class CheckContentTransformer extends MockTransformer {
+  final String content;
+  final String addition;
+
+  CheckContentTransformer(this.content, this.addition);
+
+  Future<bool> doIsPrimary(Asset asset) =>
+    asset.readAsString().then((value) => value == content);
+
+  Future doApply(Transform transform) {
+    return getPrimary(transform).then((primary) {
+      return primary.readAsString().then((value) {
+        transform.addOutput(
+            new Asset.fromString(primary.id, "$value$addition"));
+      });
+    });
+  }
+}