Add Hashable to Date interface.

frog/lib/date_implementation.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.
 
 // TODO(jmesserly): the native class should be the real JS Date.
 // TODO(jimhug): Making the date value non-lazy might be a good path there.
 class DateImplementation implements Date {
   final int value;
   final TimeZone timeZone;
 
@@ skipping 100 matching lines @@
 
   bool operator ==(other) {
     if (!(other is DateImplementation)) return false;
     return (value == other.value) && (timeZone == other.timeZone);
   }
 
   int compareTo(Date other) {
     return value.compareTo(other.value);
   }
 
+  int hashCode() {
+    return value;
+  }
+
   Date changeTimeZone(TimeZone targetTimeZone) {
     if (targetTimeZone == null) {
       targetTimeZone = new TimeZoneImplementation.local();
     }
     return new Date.fromEpoch(value, targetTimeZone);
   }
 
   int get year() native
   '''return this.isUtc() ? this._asJs().getUTCFullYear() :
     this._asJs().getFullYear();''' {
@@ skipping 150 matching lines @@
     return isUtc == other.isUtc;
   }
 
   String toString() {
     if (isUtc) return "TimeZone (UTC)";
     return "TimeZone (Local)";
   }
 
   final bool isUtc;
 }