Add comparison operators to Date class.

lib/compiler/implementation/lib/mockimpl.dart

Index: lib/compiler/implementation/lib/mockimpl.dart
diff --git a/lib/compiler/implementation/lib/mockimpl.dart b/lib/compiler/implementation/lib/mockimpl.dart
index 8f24860cb81145857ec81091823fca41980e7266..2a6171f7adc2f2763a0c80bd84671d385692f1cf 100644
--- a/lib/compiler/implementation/lib/mockimpl.dart
+++ b/lib/compiler/implementation/lib/mockimpl.dart
@@ -278,10 +278,15 @@ class DateImplementation implements Date {
     return (value == other.value) && (timeZone == other.timeZone);
   }
 
-  int compareTo(Date other) {
-    checkNull(other);
-    return value.compareTo(other.value);
-  }
+  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;