follow up for #25578, add `@checked` to package:meta

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 a9424f9536c996b1dc40ec1d261ea211d70f1ca3..633a4d362fb50fe80e1fd26023b6ad05b5db415b 100644
--- a/pkg/meta/lib/meta.dart
+++ b/pkg/meta/lib/meta.dart
@@ -128,6 +128,19 @@ class Required {
   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();
+
+class _Checked {
+  const _Checked();
+}
+
 class _Factory {
   const _Factory();
 }