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

pkg/front_end/testcases/runtime_checks/covariant_generic_parameter_in_interface.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;
+
+abstract class I<T> {
+  void f1(T x);
+  void f2(T x);
+}
+
+class C<U> implements I<int> {
+  void f1(int /*@checkFormal=semiSafe*/ x) {}
+  void f2(int /*@checkFormal=semiSafe*/ x, [U /*@checkFormal=semiSafe*/ y]) {}
+}
+
+void g1(C<num> c) {
+  c.f1(1);
+}
+
+void g2(I<num> i) {
+  i.f1 /*@checkCall=interface(semiTyped:0)*/ (1.5);
+}
+
+void g3(C<num> c) {
+  c.f2 /*@checkCall=interface(semiTyped:1)*/ (1, 1.5);
+}
+
+void test() {
+  g2(new C<num>());
+}
+
+void main() {}