fixes #27385, implement virtual fields in DDC

tests/language_strong/override_inheritance_field_test.dart

 // Copyright (c) 2014, 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:meta/meta.dart" show virtual;
+
 class A {
   get getter1 => null; /// 01: ok
   num get getter2 => null; /// 02: ok
   num get getter3 => null; /// 03: ok
   int get getter4 => null; /// 04: ok
   int get getter5 => null; /// 05: static type warning
   int get getter6 => null; /// 06: ok
   int get getter7 => null; /// 07: static type warning
   int get getter8 => null; /// 08: static type warning
 
   set setter1(_) => null; /// 21: ok
   void set setter2(_) {} /// 22: ok
   set setter3(_) => null; /// 23: ok
   set setter4(_) => null; /// 24: ok
   set setter5(num _) => null; /// 25: ok
   set setter6(num _) => null; /// 26: ok
   set setter7(int _) => null; /// 27: ok
   set setter8(int _) => null; /// 28: static type warning
   set setter9(int _) => null; /// 29: ok
   set setter10(int _) => null; /// 30: static type warning
   set setter11(int _) => null; /// 31: static type warning
 
-  int field1; /// 41: ok
+  @virtual int field1; /// 41: ok
   num field2; /// 42: ok
   int field3; /// 43: ok
   int field4; /// 44: static type warning
   int field5; /// 45: ok
-  num field6; /// 46: ok
+  @virtual num field6; /// 46: ok
   num field7; /// 47: static type warning
   num get field8 => null; /// 48: static type warning
   num field9; /// 49: ok
   num field10; /// 50: ok
   set field11(int _) {} /// 51: ok
   void set field12(int _) {} /// 52: ok
   num field13; /// 53: static type warning
   set field14(num _) {} /// 54: static type warning
 }
 
@@ skipping 71 matching lines @@
   double get getter10 => null; /// 10: continued
   String get getter11 => null; /// 11: continued
   set setter13(double _) => null; /// 33: continued
   set setter13(double _) => null; /// 33a: continued
   set setter14(String _) => null; /// 34: continued
 }
 
 main() {
   new SubClass();
 }