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

pkg/front_end/testcases/runtime_checks/covariant_generic_method_type_parameter.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;
+
+class C<T> {
+  void f< /*@checkFormal=semiSafe*/ U extends T>(U x) {}
+  void g1< /*@checkFormal=semiSafe*/ U extends T>() {
+    this.f<U>(1.5);
+  }
+}
+
+void g2(C<Object> c) {
+  c.f<num> /*@checkCall=interface(semiTyped:<0>)*/ (1.5);
+}
+
+void test() {
+  new C<int>().g1<num> /*@checkCall=interface(semiTyped:<0>)*/ ();
+  g2(new C<int>()); // runtime error
+}
+
+void main() {}