fixes #27385, implement virtual fields in DDC

tests/language_strong/super_field_test.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 // Dart test for testing super field access.
 
 import "package:expect/expect.dart";
-
+import "package:meta/meta.dart" show virtual;
 
 class A {
   A() {
     city = "Bern";
   }
   String greeting() {
     return "Gruezi";
   }
-  String city;
+  @virtual String city;
 }
 
 
 class B extends A {
   B() : super() {}
   String greeting() {
     return "Hola " + super.greeting();
   }
 }
 
@@ skipping 22 matching lines @@
     Expect.equals("Bern", b.city);
     Expect.equals("Basel Bern", c.city);
     c.city = "Zurich";
     Expect.equals("Basel Zurich", c.city);
   }
 }
 
 main() {
   SuperFieldTest.testMain();
 }