Migrated test block 220 to Dart 2.0.

tests/lib_2/mirrors/generic_superclass_test.dart

 // Copyright (c) 2013, 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';
 import 'dart:mirrors';
 
 class A<T> {}
 
 class B extends A<U> {}
@@ skipping 109 matching lines @@
 void testObject() {
   ClassMirror object = reflectClass(Object);
   Expect.equals(null, object.superclass);
 }
 
 main() {
   testOriginals();
   testInstances();
   testObject();
 }