Migrate test block 113 to Dart 2.0.

tests/language/generic_field_mixin2_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> {
-  var field = 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);
-}