Refactor the barback tests somewhat.

pkg/barback/test/transformer/many_to_one.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 uses the contents of a file to define the other inputs.
+///
+/// Outputs a file with the same name as the primary but with an "out"
+/// extension containing the concatenated contents of all non-primary inputs.
+class ManyToOneTransformer 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 input
+  /// files at each of those paths.
+  ManyToOneTransformer(this.extension);
+
+  Future<bool> doIsPrimary(Asset asset) =>
+    new Future.value(asset.id.extension == ".$extension");
+
+  Future doApply(Transform transform) {
+    return getPrimary(transform)
+        .then((primary) => primary.readAsString())
+        .then((contents) {
+      // Get all of the included inputs.
+      return Future.wait(contents.split(",").map((path) {
+        var id = new AssetId(transform.primaryId.package, path);
+        return getInput(transform, id).then((input) => input.readAsString());
+      }));
+    }).then((outputs) {
+      var id = transform.primaryId.changeExtension(".out");
+      transform.addOutput(new Asset.fromString(id, outputs.join()));
+    });
+  }
+
+  String toString() => "many->1 $extension";
+}