Add comparison operators to Date class.

runtime/lib/date.dart

Index: runtime/lib/date.dart
diff --git a/runtime/lib/date.dart b/runtime/lib/date.dart
index c670ab32ac41c05326605c4448db9eaf4c7c8509..8652125e3a3132a8ae7d1a0e6418857fd73df956 100644
--- a/runtime/lib/date.dart
+++ b/runtime/lib/date.dart
@@ -112,6 +112,14 @@ class DateImplementation implements Date {
     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) => value.compareTo(other.value);
   int hashCode() => value;