Don't require Transformer methods to return Futures.

pkg/barback/test/package_graph/transform/transform_test.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.
 
 // This library contains tests for transformer behavior that relates to actions
 // happening concurrently or other complex asynchronous timing behavior.
 library barback.test.package_graph.transform.transform_test;
 
 import 'package:barback/src/utils.dart';
 import 'package:scheduled_test/scheduled_test.dart';
@@ skipping 324 matching lines @@
     expectAsset('app|foo.out', 'spread txt.out');
     buildShouldSucceed();
 
     modifyAsset("app|foo.txt", "bar.mid");
     updateSources(["app|foo.txt"]);
     expectNoAsset('app|foo.out');
     expectAsset('app|bar.out', 'spread txt.out');
     buildShouldSucceed();
   });
 
+  test("gets an asset transformed by a sync transformer", () {
+    initGraph(["app|foo.blub"], {"app": [
+      [new SyncRewriteTransformer("blub", "blab")]
+    ]});
+    updateSources(["app|foo.blub"]);
+    expectAsset("app|foo.blab", "new.blab");
+    buildShouldSucceed();
+  });
+
   group("Transform.hasInput", () {
     test("returns whether an input exists", () {
       initGraph(["app|foo.txt", "app|bar.txt"], {'app': [
         [new HasInputTransformer(['app|foo.txt', 'app|bar.txt', 'app|baz.txt'])]
       ]});
 
       updateSources(['app|foo.txt', 'app|bar.txt']);
       expectAsset('app|foo.txt',
           'app|foo.txt: true, app|bar.txt: true, app|baz.txt: false');
       buildShouldSucceed();
@@ skipping 21 matching lines @@
       updateSources(['app|foo.txt']);
       expectAsset('app|foo.txt', 'app|bar.txt: false');
       buildShouldSucceed();
     
       updateSources(['app|bar.txt']);
       expectAsset('app|foo.txt', 'app|bar.txt: true');
       buildShouldSucceed();
     });
   });
 }