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

pkg/front_end/testcases/runtime_checks/contravariant_generic_return_tear_off.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 void F<T>(T x);
+typedef F<T> G<T>();
+
+class C<T> {
+  F<T> _x;
+  C(this._x);
+  F<T> f() => _x;
+}
+
+G<num> g(C<num> c) {
+  return c. /*@checkTearOff=() -> (num) -> void*/ f;
+}
+
+void h(int i) {
+  print('$i');
+}
+
+void test() {
+  var x = g(new C<int>(h));
+}
+
+void main() {}