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

pkg/polymer/lib/src/build/mirrors_remover.dart

Index: pkg/polymer/lib/src/build/mirrors_remover.dart
diff --git a/pkg/polymer/lib/src/build/mirrors_remover.dart b/pkg/polymer/lib/src/build/mirrors_remover.dart
index 99677bedbaf48307a3f9310ea58a45b3a96f8ccf..da73b11a496d53ab57086c21713ec821467b6fe2 100644
--- a/pkg/polymer/lib/src/build/mirrors_remover.dart
+++ b/pkg/polymer/lib/src/build/mirrors_remover.dart
@@ -14,9 +14,8 @@ class MirrorsRemover extends Transformer {
   MirrorsRemover.asPlugin();
 
   /// Only apply to `lib/polymer.dart`.
-  Future<bool> isPrimary(Asset input) => new Future.value(
-      input.id.package == 'polymer' &&
-      input.id.path == 'lib/polymer.dart');
+  Future<bool> isPrimary(AssetId id) => new Future.value(
+      id.package == 'polymer' && id.path == 'lib/polymer.dart');
 
   Future apply(Transform transform) {
     var id = transform.primaryInput.id;