Don't require Transformer methods to return Futures.

pkg/barback/test/transformer/emit_nothing.dart

Index: pkg/barback/test/transformer/emit_nothing.dart
diff --git a/pkg/barback/test/transformer/emit_nothing.dart b/pkg/barback/test/transformer/emit_nothing.dart
index a414cb891733c5a6dfafd040e5d5ff35f3f816dd..9cf513446ea228c15ef205fb32abd188d2749dfa 100644
--- a/pkg/barback/test/transformer/emit_nothing.dart
+++ b/pkg/barback/test/transformer/emit_nothing.dart
@@ -4,8 +4,6 @@
 
 library barback.test.transformer.emit_nothing;
 
-import 'dart:async';
-
 import 'package:barback/barback.dart';
 
 import 'mock.dart';
@@ -16,10 +14,11 @@ class EmitNothingTransformer extends MockTransformer {
 
   EmitNothingTransformer(this.extension);
 
-  Future<bool> doIsPrimary(AssetId id) =>
-    new Future.value(id.extension == ".$extension");
+  bool doIsPrimary(AssetId id) => id.extension == ".$extension";
 
-  Future doApply(Transform transform) => new Future.value();
+  void doApply(Transform transform) {
+    // Emit nothing.
+  }
 
   String toString() => "$extension->nothing";
 }