Don't require Transformer methods to return Futures.

pkg/barback/test/package_graph/transform/transform_test.dart

Index: pkg/barback/test/package_graph/transform/transform_test.dart
diff --git a/pkg/barback/test/package_graph/transform/transform_test.dart b/pkg/barback/test/package_graph/transform/transform_test.dart
index dce5e2cd15586ea2cc38cdca8b26b81f232b88ec..72d21237ebf08b7abb8a8317f34ebeedbf313b73 100644
--- a/pkg/barback/test/package_graph/transform/transform_test.dart
+++ b/pkg/barback/test/package_graph/transform/transform_test.dart
@@ -342,6 +342,15 @@ main() {
     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': [