Add Hashable to Date interface.

frog/lib/date_implementation.dart

Index: frog/lib/date_implementation.dart
diff --git a/frog/lib/date_implementation.dart b/frog/lib/date_implementation.dart
index c3886319201fba95fe298ab39c748aec9fe64baa..76cfd1825c98b49fe10d6901ea3887fe96971b5e 100644
--- a/frog/lib/date_implementation.dart
+++ b/frog/lib/date_implementation.dart
@@ -118,6 +118,10 @@ class DateImplementation implements Date {
     return value.compareTo(other.value);
   }
 
+  int hashCode() {
+    return value;
+  }
+
   Date changeTimeZone(TimeZone targetTimeZone) {
     if (targetTimeZone == null) {
       targetTimeZone = new TimeZoneImplementation.local();