Don't require Transformer methods to return Futures.

pkg/barback/test/transformer/create_asset.dart

Index: pkg/barback/test/transformer/create_asset.dart
diff --git a/pkg/barback/test/transformer/create_asset.dart b/pkg/barback/test/transformer/create_asset.dart
index 5db8940fd7226c45615858c098edaf557084e1aa..82e80b37d4ba8a3e681ae5066126aa5482800c6d 100644
--- a/pkg/barback/test/transformer/create_asset.dart
+++ b/pkg/barback/test/transformer/create_asset.dart
@@ -4,10 +4,7 @@
 
 library barback.test.transformer.create_asset;
 
-import 'dart:async';
-
 import 'package:barback/barback.dart';
-import 'package:barback/src/utils.dart';
 
 import 'mock.dart';
 
@@ -17,12 +14,10 @@ class CreateAssetTransformer extends MockTransformer {
 
   CreateAssetTransformer(this.output);
 
-  Future<bool> doIsPrimary(_) => new Future.value(true);
+  bool doIsPrimary(AssetId id) => true;
 
-  Future doApply(Transform transform) {
-    return newFuture(() {
-      transform.addOutput(
-          new Asset.fromString(new AssetId.parse(output), output));
-    });
+  void doApply(Transform transform) {
+    transform.addOutput(
+        new Asset.fromString(new AssetId.parse(output), output));
   }
 }