Deprecate the "virtual" annotation

Change-Id: I3fa88a70f36c769be3649ea7d40d280e6cf6b633
Reviewed-on: https://dart-review.googlesource.com/c/84443
Reviewed-by: Nate Bosch <nbosch@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>
diff --git a/pkg/meta/lib/meta.dart b/pkg/meta/lib/meta.dart
index 38a8283..0b1699f 100644
--- a/pkg/meta/lib/meta.dart
+++ b/pkg/meta/lib/meta.dart
@@ -203,6 +203,11 @@
 const _Sealed sealed = const _Sealed();
 
 /// Used to annotate a field that is allowed to be overridden in Strong Mode.
+///
+/// Deprecated: Most of strong mode is now the default in 2.0, but the notion of
+/// virtual fields was dropped, so this annotation no longer has any meaning.
+/// Uses of the annotation should be removed.
+@deprecated
 const _Virtual virtual = const _Virtual();
 
 /// Used to annotate an instance member that was made public so that it could be
@@ -302,6 +307,7 @@
   const _Sealed();
 }
 
+@deprecated
 class _Virtual {
   const _Virtual();
 }