Migrated test block 220 to Dart 2.0.

tests/lib_2/mirrors/generic_mixin_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.
 
 library test.generic_mixin;
 
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
@@ skipping 162 matching lines @@
       [reflectClass(String)]);
   typeArguments(
       reflect(new GenericMultipleMixins<bool, String, int>())
           .type
           .superclass
           .superclass
           .superclass
           .mixin,
       [reflectClass(bool)]);
 }