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

pkg/front_end/testcases/runtime_checks/covariant_generic_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(T /*@checkFormal=semiSafe*/ x) {}
+}
+
+void g1(C<num> c) {
+  c.f /*@checkCall=interface(semiTyped:0)*/ (1.5);
+}
+
+void g2(C<int> c) {
+  c.f /*@checkCall=interface(semiTyped:0)*/ (1);
+}
+
+void g3(C<num> c) {
+  c.f /*@checkCall=interface(semiTyped:0)*/ (null);
+}
+
+main() {}