Stop using the Hashable interface.

lib/core/date.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.
 
 // Dart core library.
 
 /**
  * Date is the public interface to a point in time.
  *
  * It can represent time values that are at a distance of at most
  * 8,640,000,000,000,000ms (100,000,000 days) from epoch (1970-01-01 UTC). In
  * other words: [:millisecondsSinceEpoch.abs() <= 8640000000000000:].
  *
  * Also see [Stopwatch] for means to measure time-spans.
  */
-abstract class Date implements Comparable, Hashable {
+abstract class Date implements Comparable {
   // Weekday constants that are returned by [weekday] method:
   static const int MON = 1;
   static const int TUE = 2;
   static const int WED = 3;
   static const int THU = 4;
   static const int FRI = 5;
   static const int SAT = 6;
   static const int SUN = 7;
   static const int DAYS_IN_WEEK = 7;
 
@@ skipping 189 matching lines @@
   /**
    * Returns a new [Date] with the [duration] subtracted from this instance.
    */
   Date subtract(Duration duration);
 
   /**
    * Returns a [Duration] with the difference of [:this:] and [other].
    */
   Duration difference(Date other);
 }