Refactor the barback tests somewhat.

pkg/barback/test/transformer/one_to_many.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 takes an input asset that contains a comma-separated
+/// list of paths and outputs a file for each path.
+class OneToManyTransformer extends MockTransformer {
+  final String extension;
+
+  /// Creates a transformer that consumes assets with [extension].
+  ///
+  /// That file contains a comma-separated list of paths and it will output
+  /// files at each of those paths.
+  OneToManyTransformer(this.extension);
+
+  Future<bool> doIsPrimary(Asset asset) =>
+    new Future.value(asset.id.extension == ".$extension");
+
+  Future doApply(Transform transform) {
+    return getPrimary(transform)
+        .then((input) => input.readAsString())
+        .then((lines) {
+      for (var line in lines.split(",")) {
+        var id = new AssetId(transform.primaryId.package, line);
+        transform.addOutput(new Asset.fromString(id, "spread $extension"));
+      }
+    });
+  }
+
+  String toString() => "1->many $extension";
+}