Only pass an AssetId to isPrimary and declareOutputs.

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 60b9e2b2d1c05b23e0956c9fb3b34c37d75a2e92..f62fa2c4fde1c05027247e3267b33b88af7bc770 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;