support `@virtual` fields, fix #27384

pkg/meta/lib/meta.dart

Index: pkg/meta/lib/meta.dart
diff --git a/pkg/meta/lib/meta.dart b/pkg/meta/lib/meta.dart
index 633a4d362fb50fe80e1fd26023b6ad05b5db415b..984287546f6558dfa0ef1395c7950449d28d840d 100644
--- a/pkg/meta/lib/meta.dart
+++ b/pkg/meta/lib/meta.dart
@@ -128,7 +128,6 @@ class Required {
   const Required([this.reason]);
 }
 
-
 /// Used to annotate a parameter of an instance method that overrides another
 /// method.
 ///
@@ -137,6 +136,9 @@ class Required {
 /// 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();
 }
@@ -161,6 +163,10 @@ class _Protected {
   const _Protected();
 }
 
+class _Virtual {
+  const _Virtual();
+}
+
 class _VisibleForTesting {
   const _VisibleForTesting();
 }