support `@virtual` fields, fix #27384

pkg/meta/lib/meta.dart

 // Copyright (c) 2016, 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.
 
 /// Constants for use in metadata annotations.
 ///
 /// See also `@deprecated` and `@override` in the `dart:core` library.
 ///
 /// Annotations provide semantic information that tools can use to provide a
 /// better user experience. For example, an IDE might not autocomplete the name
@@ skipping 110 matching lines @@
   ///         @Required('Buttons must do something when pressed')
   ///         Function onPressed,
   ///         ...
   ///     }) ...
   final String reason;
 
   /// Initialize a newly created instance to have the given [reason].
   const Required([this.reason]);
 }
 
-
 /// Used to annotate a parameter of an instance method that overrides another
 /// method.
 ///
 /// Indicates that this parameter may have a tighter type than the parameter on
 /// its superclass. The actual argument will be checked at runtime to ensure it
 /// is a subtype of the overridden parameter type.
 const _Checked checked = const _Checked();
 
+/// Used to annotate a field is allowed to be overridden in Strong Mode.
+const _Virtual virtual = const _Virtual();
+
 class _Checked {
   const _Checked();
 }
 
 class _Factory {
   const _Factory();
 }
 
 class _Literal {
   const _Literal();
 }
 
 class _MustCallSuper {
   const _MustCallSuper();
 }
 
 class _OptionalTypeArgs {
   const _OptionalTypeArgs();
 }
 
 class _Protected {
   const _Protected();
 }
 
+class _Virtual {
+  const _Virtual();
+}
+
 class _VisibleForTesting {
   const _VisibleForTesting();
 }