Migrate test block 5 to Dart 2.0.

tests/corelib_strong/date_time2_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";
-
-// Dart test program for DateTime's hashCode.
-
-main() {
-  var d = DateTime.parse("2000-01-01T00:00:00Z");
-  var d2 = DateTime.parse("2000-01-01T00:00:01Z");
-  // There is no guarantee that the hashcode for these two dates is different,
-  // but in the worst case we will have to fix this test.
-  // The important test here is, that DateTime .
-  Expect.isFalse(d.hashCode == d2.hashCode);
-}