Beef up the covariant generic parameter tear off test case

pkg/front_end/testcases/runtime_checks/covariant_generic_parameter_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 U G<T, U>(T x);
+
 class C<T> {
-  void f(T /*@checkFormal=semiSafe*/ x) {}
+  void f1(T /*@checkFormal=semiSafe*/ x) {}
+  T f2(List<T> /*@checkFormal=semiSafe*/ x) => x.first;
 }
 
-void g1(C<num> c) {
-  c.f;
+F<num> g1(C<num> c) {
+  return c.f1;
 }
 
 void g2(C<int> c, Object x) {
   F<Object> f = g1(c) as F<Object>;
   f /*@checkCall=interface(semiTyped:0)*/ (x);
 }
 
+G<List<num>, num> g3(C<num> c) {
+  return c.f2;
+}
+
 void test() {
   var x = g1(new C<int>());
   x /*@checkCall=interface(semiTyped:0)*/ (1.5);
+  g3(new C<int>());
 }
 
 main() {}