Migrate test block 5 to Dart 2.0.

tests/corelib_2/date_time9_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.
 
 import "package:expect/expect.dart";
 
 main() {
   var dt = new DateTime.now();
   Expect.isTrue(dt is Comparable);
 
@@ skipping 22 matching lines @@
   Expect.isTrue(dt2.compareTo(dt3b) < 0);
   Expect.isTrue(dt4.compareTo(dt5b) < 0);
 
   Expect.isTrue(dt3.compareTo(dt2) > 0);
   Expect.isTrue(dt4.compareTo(dt3) > 0);
   Expect.isTrue(dt5.compareTo(dt4) > 0);
 
   Expect.isTrue(dt3b.compareTo(dt2) > 0);
   Expect.isTrue(dt5b.compareTo(dt4) > 0);
 }