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

R=pquitslund@google.com

Review URL: https://codereview.chromium.org/2334413002 .

CHANGELOG.md

@@ -39,6 +39,25 @@
     }
     ```
 
+* New feature - use `@checked` to override a method and tighten a parameter
+    type (SDK issue [25578](https://github.com/dart-lang/sdk/issues/25578)).
+
+    ```dart
+    import 'package:meta/meta.dart' show checked;
+    class View {
+      addChild(View v) {}
+    }
+    class MyView extends View {
+      // this override is legal, it will check at runtime if we actually
+      // got a MyView.
+      addChild(@checked MyView v) {}
+    }
+    main() {
+      dynamic mv = new MyView();
+      mv.addChild(new View()); // runtime error
+    }
+    ```
+
 ## 1.19.0
 
 ### Language changes

pkg/meta/CHANGELOG.md

@@ -1,3 +1,23 @@
+## 1.0.3
+* Introduce `@checked` to override a method and tighten a parameter
+    type (SDK issue [25578](https://github.com/dart-lang/sdk/issues/25578)).
+
+    ```dart
+    import 'package:meta/meta.dart' show checked;
+    class View {
+      addChild(View v) {}
+    }
+    class MyView extends View {
+      // this override is legal, it will check at runtime if we actually
+      // got a MyView.
+      addChild(@checked MyView v) {}
+    }
+    main() {
+      dynamic mv = new MyView();
+      mv.addChild(new View()); // runtime error
+    }
+    ```
+
 ## 1.0.2
 * Introduce `@visibleForTesting` annotation for declarations that may be referenced only in the library or in a test.
 

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();
 }

pkg/meta/pubspec.yaml

@@ -1,5 +1,5 @@
 name: meta
-version: 1.0.2
+version: 1.0.3
 author: Dart Team <misc@dartlang.org>
 homepage: http://www.dartlang.org
 description: >