Migrate test block 5 to Dart 2.0.

tests/corelib_strong/date_time6_test.dart

-// Copyright (c) 2012, 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";
-
-// Test DateTime comparison operators.
-
-main() {
-  var d = new DateTime.fromMillisecondsSinceEpoch(0, isUtc: true);
-  var d2 = new DateTime.fromMillisecondsSinceEpoch(1, isUtc: true);
-  Expect.isTrue(d.isBefore(d2));
-  Expect.isTrue(!d.isAfter(d2));
-  Expect.isTrue(d2.isAfter(d));
-  Expect.isTrue(!d2.isBefore(d));
-  Expect.isFalse(d2.isBefore(d));
-  Expect.isFalse(!d2.isAfter(d));
-  Expect.isFalse(d.isAfter(d2));
-  Expect.isFalse(!d.isBefore(d2));
-
-  d = new DateTime.fromMillisecondsSinceEpoch(-1, isUtc: true);
-  d2 = new DateTime.fromMillisecondsSinceEpoch(0, isUtc: true);
-  Expect.isTrue(d.isBefore(d2));
-  Expect.isTrue(!d.isAfter(d2));
-  Expect.isTrue(d2.isAfter(d));
-  Expect.isTrue(!d2.isBefore(d));
-  Expect.isFalse(d2.isBefore(d));
-  Expect.isFalse(!d2.isAfter(d));
-  Expect.isFalse(d.isAfter(d2));
-  Expect.isFalse(!d.isBefore(d2));
-}