Use "Object" as the reified type of covariant override parameters.

tests/language_strong/checked_covariant_overrides_test.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.
-
-import "package:expect/expect.dart";
-import 'package:meta/meta.dart' show checked;
-
-// Test runtime behavior of @checked
-
-class View {
-  addChild(View v) {}
-  transform(View fn(View v)) {}
-}
-class MyView extends View {
-  addChild(@checked MyView v) {}
-  transform(@checked MyView fn(Object v)) {}
-}
-
-main() {
-  dynamic mv = new MyView();
-  dynamic v = new View();
-  
-  mv.addChild(mv);
-  Expect.throws(() => mv.addChild(v));
-
-  mv.transform((_) => new MyView());
-
-  // TODO(jmesserly): these *should* be a cast failures, but DDC is currently
-  // ignoring function type failures w/ a warning at the console...
-
-  // * -> * not a subtype of Object -> MyView
-  // Expect.throws(() => mv.transform((_) => mv));
-
-  // View -> View not a subtype of Object -> MyView
-  // Expect.throws(() => mv.transform((View x) => x));
-}