fixes #27385, implement virtual fields in DDC

tests/language_strong/closure_with_super_field_test.dart

Index: tests/language_strong/closure_with_super_field_test.dart
diff --git a/tests/language_strong/closure_with_super_field_test.dart b/tests/language_strong/closure_with_super_field_test.dart
index 67f75d1befb3ce92f8f623b000444ab900faa1ab..d98470ac8ad1c290e89c6585f1d8b7474498c32a 100644
--- a/tests/language_strong/closure_with_super_field_test.dart
+++ b/tests/language_strong/closure_with_super_field_test.dart
@@ -3,9 +3,10 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import "package:expect/expect.dart";
+import "package:meta/meta.dart" show virtual;
 
 class A {
-  int a;
+  @virtual int a;
   A(): a = 42;
 }