fixes #27385, implement virtual fields in DDC

tests/language_strong/setter_override2_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.
 
 // Test that we do not report a compile-time error when an instance setter named
 // foo= is declared in a class inheriting an instance method, field, or getter
 // named foo, or an instance setter named foo=.
 
 import "package:expect/expect.dart";
+import "package:meta/meta.dart" show virtual;
 
 class A {
-  var foo = 42;  /// 00: ok
+  @virtual var foo = 42;  /// 00: ok
   get foo => 42;  /// 01: ok
   foo() => 42;  /// 02: ok
   set foo(value) { }  /// 03: ok
 }
 
 class B extends A {
   var foo_;
   set foo(value) { foo_ = value; }
 }
 
 main() {
   var b = new B();
   b.foo = 42;
   Expect.equals(42, b.foo_);
 }