fixes #27385, implement virtual fields in DDC

tests/language_strong/closure_with_super_field_test.dart

 // Copyright (c) 2012, 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.
 
 import "package:expect/expect.dart";
+import "package:meta/meta.dart" show virtual;
 
 class A {
-  int a;
+  @virtual int a;
   A(): a = 42;
 }
 
 class B extends A {
   int get a => 54;
   returnSuper() => super.a;
   returnSuperInClosure() => () => super.a;
 }
 
 main() {
   B b = new B();
   Expect.equals(54, b.a);
   Expect.equals(42, b.returnSuper());
   Expect.equals(42, b.returnSuperInClosure()());
 }