Refactor the barback tests somewhat.

pkg/barback/test/transformer/rewrite.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 assets ending with one extension and generates
+/// assets with a given extension.
+///
+/// Appends the output extension to the contents of the input file.
+class RewriteTransformer extends MockTransformer {
+  final String from;
+  final String to;
+
+  /// Creates a transformer that rewrites assets whose extension is [from] to
+  /// one whose extension is [to].
+  ///
+  /// [to] may be a space-separated list in which case multiple outputs will be
+  /// created for each input.
+  RewriteTransformer(this.from, this.to);
+
+  Future<bool> doIsPrimary(Asset asset) =>
+    new Future.value(asset.id.extension == ".$from");
+
+  Future doApply(Transform transform) {
+    return getPrimary(transform).then((input) {
+      return Future.wait(to.split(" ").map((extension) {
+        var id = transform.primaryId.changeExtension(".$extension");
+        return input.readAsString().then((content) {
+          transform.addOutput(new Asset.fromString(id, "$content.$extension"));
+        });
+      }));
+    });
+  }
+
+  String toString() => "$from->$to";
+}