Begin writing tests of the runtime checks needed for Dart 2.0 soundness.

pkg/front_end/testcases/runtime_checks/covariant_keyword.dart

+// Copyright (c) 2017, 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.
+
+/*@testedFeatures=checks*/
+library test;
+
+typedef F<T>(T x);
+
+class C {
+  void f(num x) {}
+}
+
+class D extends C {
+  void f(covariant int /*@checkFormal=unsafe*/ x) {}
+}
+
+class E extends D {
+  void f(int /*@checkFormal=unsafe*/ x) {}
+}
+
+void g1(C c) {
+  c.f(1.5);
+}
+
+F<num> g2(C c) {
+  return c.f;
+}
+
+test() {
+  g1(new D());
+  F<num> x = g2(new D());
+}
+
+main() {}