Add support for transformer clusters to barback.

pkg/barback/test/utils.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.
 
 library barback.test.utils;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert' show Encoding;
 
@@ skipping 114 matching lines @@
 
 /// Removes [assets] from the current [PackageProvider].
 ///
 /// Each item in the list may either be an [AssetId] or a string that can be
 /// parsed as one. Unlike [removeSources], this is not automatically scheduled
 /// and will be run synchronously when called.
 void removeSourcesSync(Iterable assets) =>
     _barback.removeSources(_parseAssets(assets));
 
 /// Sets the transformers for [package] to [transformers].
-void updateTransformers(String package,
-    Iterable<Iterable<Transformer>> transformers) {
+void updateTransformers(String package, Iterable<Iterable> transformers) {
   schedule(() => _barback.updateTransformers(package, transformers),
       "updating transformers for $package");
 }
 
 /// Parse a list of strings or [AssetId]s into a list of [AssetId]s.
 List<AssetId> _parseAssets(Iterable assets) {
   return assets.map((asset) {
     if (asset is String) return new AssetId.parse(asset);
     return asset;
   }).toList();
@@ skipping 372 matching lines @@
 
   _MockAsset(this.id, this.contents);
 
   Future<String> readAsString({Encoding encoding}) =>
       new Future.value(contents);
 
   Stream<List<int>> read() => throw new UnimplementedError();
 
   String toString() => "MockAsset $id $contents";
 }