fixes #27385, implement virtual fields in DDC

tests/language_strong/getters_setters_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 GettersSettersTest {
 
   static int foo;
 
   static testMain() {
     A a = new A();
     a.x = 2;
     Expect.equals(2, a.x);
     Expect.equals(2, a.x_);
@@ skipping 38 matching lines @@
 
     ReferenceField rf = new ReferenceField();
     rf.x_ = 1;
     Expect.equals(1, rf.getIt());
     rf.setIt(2);
     Expect.equals(2, rf.x_);
   }
 }
 
 class A {
-  int x_;
+  @virtual int x_;
   static int foo;
 
   static get bar {
     return foo;
   }
 
   static set bar(newValue) {
     foo = newValue;
   }
 
@@ skipping 67 matching lines @@
   }
 
   int getIt() {
      return super.x_;
   }
 }
 
 main() {
   GettersSettersTest.testMain();
 }