Add comparison operators to Date class.

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 86 matching lines @@
     }
   }
 
   const DateImplementation.fromEpoch(this.value, this.timeZone);
 
   bool operator ==(other) {
     if (!(other is DateImplementation)) return false;
     return (value == other.value) && (timeZone == other.timeZone);
   }
 
+  bool operator <(Date other) => value < other.value;
+
+  bool operator <=(Date other) => value <= other.value;
+
+  bool operator >(Date other) => value > other.value;
+
+  bool operator >=(Date other) => value >= other.value;
+
   int compareTo(Date other) {
     return value.compareTo(other.value);
   }
 
   int hashCode() {
     return value;
   }
 
   Date changeTimeZone(TimeZone targetTimeZone) {
     if (targetTimeZone == null) {
@@ skipping 158 matching lines @@
     return isUtc == other.isUtc;
   }
 
   String toString() {
     if (isUtc) return "TimeZone (UTC)";
     return "TimeZone (Local)";
   }
 
   final bool isUtc;
 }