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

pkg/front_end/testcases/runtime_checks/contravariant_generic_return.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);
+
+class C<T> {
+  F<T> f1() {}
+  List<F<T>> f2() {
+    return [this.f1()];
+  }
+}
+
+void g1(C<num> c) {
+  var x = c.f1 /*@checkReturn=(num) -> void*/ ();
+  print('hello');
+  x /*@checkCall=interface(semiTyped:0)*/ (1.5);
+}
+
+void g2(C<num> c) {
+  F<int> x = c.f1 /*@checkReturn=(num) -> void*/ ();
+  x /*@checkCall=interface(semiTyped:0)*/ (1);
+}
+
+void g3(C<num> c) {
+  var x = c.f2 /*@checkReturn=List<(num) -> void>*/ ();
+}
+
+void main() {}