fixes #27385, implement virtual fields in DDC

tests/language_strong/setter_override2_test.dart

Index: tests/language_strong/setter_override2_test.dart
diff --git a/tests/language_strong/setter_override2_test.dart b/tests/language_strong/setter_override2_test.dart
index e5ac9d09a066e8d4ff4e163612a08ca77d1003d7..d54f747c8e05bcbf210cd5b38a37dbe991690b0b 100644
--- a/tests/language_strong/setter_override2_test.dart
+++ b/tests/language_strong/setter_override2_test.dart
@@ -7,9 +7,10 @@
 // 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