fix #27353, support `@checked` covariant parameters in DDC

pkg/dev_compiler/test/codegen/language/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));
+}