fixes #27385, implement virtual fields in DDC

tests/language_strong/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 program for testing setting/getting of instance fields.
 
 import "package:expect/expect.dart";
+import "package:meta/meta.dart" show virtual;
 
 class First {
   First() {}
-  var a;
-  var b;
+  @virtual var a;
+  @virtual var b;
 
   addFields() {
     return a + b;
   }
 
   setValues() {
     a = 24;
     b = 10;
     return a + b;
   }
@@ skipping 42 matching lines @@
   static testMain() {
     // FieldTest.one();
     FieldTest.two();
   }
 }
 
 
 main() {
   FieldTest.testMain();
 }