Don't require Transformer methods to return Futures.

pkg/barback/test/transformer/sync_rewrite.dart

+// Copyright (c) 2014, 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.transformer.sync_rewrite;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+/// Like [DeclaringRewriteTransformer], but with no methods returning Futures.
+class SyncRewriteTransformer extends Transformer
+    implements DeclaringTransformer {
+  final String from;
+  final String to;
+
+  SyncRewriteTransformer(this.from, this.to);
+
+  bool isPrimary(AssetId id) => id.extension == ".$from";
+
+  void apply(Transform transform) {
+    for (var extension in to.split(" ")) {
+      var id = transform.primaryInput.id.changeExtension(".$extension");
+      transform.addOutput(new Asset.fromString(id, "new.$extension"));
+    }
+  }
+
+  void declareOutputs(DeclaringTransform transform) {
+    for (var extension in to.split(" ")) {
+      var id = transform.primaryId.changeExtension(".$extension");
+      transform.declareOutput(id);
+    }
+  }
+
+  String toString() => "$from->$to";
+}