Add test of unittests.

tests/compiler/dart2js/flatten_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 subtype_test;
+library flatten_test;
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'type_test_helper.dart';
 import 'package:compiler/src/dart_types.dart';
 import "package:compiler/src/elements/elements.dart"
        show Element, ClassElement;
 
 void main() {
   asyncTest(() => TypeEnvironment.create(r"""
@@ skipping 57 matching lines @@
     // flatten(Future<F<int>>) = int
     check(instantiate(Future_, [F_int]), int_);
 
     // flatten(Future<G<int>>) = int
     check(instantiate(Future_, [G_int]), G_int);
 
     // flatten(Future<H<int>>) = int
     check(instantiate(Future_, [H_int]), H_H_int);
   }));
 }