Handle `new Object()` in compile_from_dill_test

pkg/compiler/lib/src/js_backend/mirrors_data.dart

Index: pkg/compiler/lib/src/js_backend/mirrors_data.dart
diff --git a/pkg/compiler/lib/src/js_backend/mirrors_data.dart b/pkg/compiler/lib/src/js_backend/mirrors_data.dart
index 62c947b477c39b824cb8273de4b8eb5503a0cb7b..0ceaafcaca455f2462a7c57f581d40c8a42117c9 100644
--- a/pkg/compiler/lib/src/js_backend/mirrors_data.dart
+++ b/pkg/compiler/lib/src/js_backend/mirrors_data.dart
@@ -227,11 +227,11 @@ class MirrorsDataImpl implements MirrorsData, MirrorsDataBuilder {
     }
   }
 
-  bool shouldRetainGetter(FieldElement element) {
+  bool shouldRetainGetter(FieldEntity element) {
     return isTreeShakingDisabled && isMemberAccessibleByReflection(element);
   }
 
-  bool shouldRetainSetter(FieldElement element) {
+  bool shouldRetainSetter(FieldEntity element) {
     return isTreeShakingDisabled && isMemberAccessibleByReflection(element);
   }