fixes #27385, implement virtual fields in DDC

tests/language_strong/super_field_2_test.dart

 // Copyright (c) 2013, 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 {
-  var foo;
+  @virtual var foo;
   A(this.foo);
 
   B_Sfoo() => 'A.B_Sfoo()';
 }
 
 class B extends A {
   B(x) : super(x);
 
   B_Sfoo() => super.foo;
   BC_Sfoo() => super.foo;
@@ skipping 60 matching lines @@
   Expect.equals('Ba', inscrutable(b).BC_Sfoo());
   Expect.equals('Ca', inscrutable(c).BC_Sfoo());
   Expect.equals('Da', inscrutable(d).BC_Sfoo());
 
   Expect.equals('Ba', inscrutable(b).BCD_Sfoo());
   Expect.equals('Ca', inscrutable(c).BCD_Sfoo());
   Expect.equals('Dc', inscrutable(d).BCD_Sfoo());
 
 
 }