Don't require Transformer methods to return Futures.

pkg/barback/test/transformer/check_content_and_rename.dart

Index: pkg/barback/test/transformer/check_content_and_rename.dart
diff --git a/pkg/barback/test/transformer/check_content_and_rename.dart b/pkg/barback/test/transformer/check_content_and_rename.dart
index 9fcdebb6c0e50a9b14c5280fc30e36afb6d10c26..6d690e40fcb3e8855ffd4b9ebeb7fa20c68cb220 100644
--- a/pkg/barback/test/transformer/check_content_and_rename.dart
+++ b/pkg/barback/test/transformer/check_content_and_rename.dart
@@ -21,8 +21,7 @@ class CheckContentAndRenameTransformer extends MockTransformer {
   CheckContentAndRenameTransformer(this.oldExtension, this.oldContent,
       this.newExtension, this.newContent);
 
-  Future<bool> doIsPrimary(AssetId id) =>
-      new Future.value(id.extension != '.$oldExtension');
+  bool doIsPrimary(AssetId id) => id.extension != '.$oldExtension';
 
   Future doApply(Transform transform) {
     return getPrimary(transform).then((input) {