Migrate test block 113 to Dart 2.0.

tests/language_2/generic_field_mixin5_test.dart

 // Copyright (c) 2014, 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.
 
 // Test that generic types in mixins are handled.
 
 import 'package:expect/expect.dart';
 
 class M<T> {
   Type field = () {
     return T;
   }();
 }
 
 class A<U> {}
 
 class C1<V> = Object with M<V>;
 class C2 = Object with M<int>;
 class C3 = Object with M<String>;
 
 main() {
   Expect.equals(int, new C1<int>().field);
   Expect.equals(String, new C1<String>().field);
 
   Expect.equals(int, new C2().field);
 
   Expect.equals(String, new C3().field);
 }