Add Hashable to Date interface.

runtime/lib/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.
 
 class TimeZoneImplementation implements TimeZone {
   const TimeZoneImplementation.utc() : isUtc = true;
   TimeZoneImplementation.local() : isUtc = false {}
 
   bool operator ==(Object other) {
@@ skipping 104 matching lines @@
   }
 
   const DateImplementation.fromEpoch(int this.value,
                                      TimeZone this.timeZone);
 
   bool operator ==(Object other) {
     if (!(other is DateImplementation)) return false;
     return value == other.value && timeZone == other.timeZone;
   }
 
-  int compareTo(Date other) {
-    return value.compareTo(other.value);
-  }
+  int compareTo(Date other) => value.compareTo(other.value);
+  int hashCode() => value;
 
   Date changeTimeZone(TimeZone targetTimeZone) {
     if (targetTimeZone === null) {
       targetTimeZone = new TimeZoneImplementation.local();
     }
     return new Date.fromEpoch(value, targetTimeZone);
   }
 
   int get year() {
     int secondsSinceEpoch = secondsSinceEpoch_;
@@ skipping 279 matching lines @@
 
   static int getHours_(int secondsSinceEpoch, bool isUtc)
       native "DateNatives_getHours";
 
   static int getMinutes_(int secondsSinceEpoch, bool isUtc)
       native "DateNatives_getMinutes";
 
   static int getSeconds_(int secondsSinceEpoch, bool isUtc)
       native "DateNatives_getSeconds";
 }