Make DateTime comparable only to DateTime.

sdk/lib/core/date_time.dart

 // Copyright (c) 2011, 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.
 
 part of dart.core;
 
 /**
  * An instant in time, such as July 20, 1969, 8:18pm GMT.
  *
  * Create a DateTime object by using one of the constructors
@@ skipping 84 matching lines @@
  * ## Other resources
  *
  * See [Duration] to represent a span of time.
  * See [Stopwatch] to measure timespans.
  *
  * The DateTime class does not provide internationalization.
  * To internationalize your code, use
  * the [intl](http://pub.dartlang.org/packages/intl) package.
  *
  */
-class DateTime implements Comparable {
+class DateTime implements Comparable<DateTime> {
   // Weekday constants that are returned by [weekday] method:
   static const int MONDAY = 1;
   static const int TUESDAY = 2;
   static const int WEDNESDAY = 3;
   static const int THURSDAY = 4;
   static const int FRIDAY = 5;
   static const int SATURDAY = 6;
   static const int SUNDAY = 7;
   static const int DAYS_PER_WEEK = 7;
 
@@ skipping 636 matching lines @@
    * In accordance with ISO 8601
    * a week starts with Monday, which has the value 1.
    *
    *     DateTime moonLanding = DateTime.parse("1969-07-20 20:18:00");
    *     assert(moonLanding.weekday == 7);
    *     assert(moonLanding.weekday == DateTime.SUNDAY);
    *
    */
   external int get weekday;
 }