Add some tests for calls with checks in strong mode.

tests/language_strong/covariant_subtyping_unsafe_call2_test.dart

 // Copyright (c) 2016, 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.
 import 'package:expect/expect.dart';
 
 class Implementation {
-  method(int x) {}
+  dynamic method(int x) {}
 }
 
 abstract class Interface<T> {
-  method(T x);
+  dynamic method(T x);
 }
 
 class Subclass extends Implementation implements Interface<int> {}
 
 main() {
   Subclass subclass = new Subclass();
   Interface<int> intInterface = subclass;
   Interface<num> numInterface = intInterface;
   Expect.throws(() => numInterface.method(2.5));
 }