Change @checked to covariant in strong tests

tests/language_strong/covariant_override/runtime_check_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)) {}
+  addChild(covariant MyView v) {}
+  transform(covariant 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
+  // TODO(jmesserly): these *should* be 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));
 }