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

pkg/front_end/testcases/runtime_checks/covariant_generic_parameter_in_interface_mixin.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 B {
+  void f(int x) {}
+}
+
+abstract class I<T> {
+  void f(T x);
+}
+
+class M {
+  void f(int x) {}
+}
+
+class /*@forwardingStub=void f(semiSafe int x)*/ C = B with M implements I<int>;
+void g1(/*safe*/ C c) {
+  c.f(1);
+}
+
+void g2(/*safe*/ I<num> i) {
+  i.f /*@checkCall=interface(semiTyped:0)*/ (1.5);
+}
+
+void test() {
+  g2(new C());
+}
+
+void main() {}