Don't require Transformer methods to return Futures.

pkg/barback/test/transformer/declaring_rewrite.dart

Index: pkg/barback/test/transformer/declaring_rewrite.dart
diff --git a/pkg/barback/test/transformer/declaring_rewrite.dart b/pkg/barback/test/transformer/declaring_rewrite.dart
index 3dd3f644b67dd6f4ce205c72237e6380d102a7aa..8ab8a15fefde00d93aef3bdcb5c51e92d500e308 100644
--- a/pkg/barback/test/transformer/declaring_rewrite.dart
+++ b/pkg/barback/test/transformer/declaring_rewrite.dart
@@ -4,8 +4,6 @@
 
 library barback.test.transformer.declaring_rewrite;
 
-import 'dart:async';
-
 import 'package:barback/barback.dart';
 
 import 'rewrite.dart';
@@ -16,7 +14,7 @@ class DeclaringRewriteTransformer extends RewriteTransformer
   DeclaringRewriteTransformer(String from, String to)
       : super(from, to);
 
-  Future declareOutputs(DeclaringTransform transform) {
+  void declareOutputs(DeclaringTransform transform) {
     if (consumePrimary) transform.consumePrimary();
     for (var extension in to.split(" ")) {
       var id = transform.primaryId.changeExtension(".$extension");