Only run [Transformer.isPrimary] once for each asset/transformer pair.

pkg/smoke/lib/src/default_transformer.dart

Index: pkg/smoke/lib/src/default_transformer.dart
diff --git a/pkg/smoke/lib/src/default_transformer.dart b/pkg/smoke/lib/src/default_transformer.dart
index 49962a9146ac5b623a12cf29b2414d7ca75a02e2..22f4e58d3640e35afab524bba15f5d781f715b23 100644
--- a/pkg/smoke/lib/src/default_transformer.dart
+++ b/pkg/smoke/lib/src/default_transformer.dart
@@ -17,9 +17,8 @@ class DefaultTransformer extends Transformer {
   DefaultTransformer.asPlugin();
 
   /// Only apply to `lib/src/implementation.dart`.
-  Future<bool> isPrimary(Asset input) => new Future.value(
-      input.id.package == 'smoke' &&
-      input.id.path == 'lib/src/implementation.dart');
+  Future<bool> isPrimary(AssetId id) => new Future.value(
+      id.package == 'smoke' && id.path == 'lib/src/implementation.dart');
 
   Future apply(Transform transform) {
     var id = transform.primaryInput.id;