Add comparison operators to Date class.

tests/corelib/date_time6_test.dart

+// Copyright (c) 2012, 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.
+
+// Test Date comparison operators.
+
+main() {
+  var d = new Date.fromEpoch(0, const TimeZone.utc());
+  var d2 = new Date.fromEpoch(1, const TimeZone.utc());
+  Expect.isTrue(d < d2);
+  Expect.isTrue(d <= d2);
+  Expect.isTrue(d2 > d);
+  Expect.isTrue(d2 >= d);
+  Expect.isFalse(d2 < d);
+  Expect.isFalse(d2 <= d);
+  Expect.isFalse(d > d2);
+  Expect.isFalse(d >= d2);
+
+  d = new Date.fromEpoch(-1, const TimeZone.utc());
+  d2 = new Date.fromEpoch(0, const TimeZone.utc());
+  Expect.isTrue(d < d2);
+  Expect.isTrue(d <= d2);
+  Expect.isTrue(d2 > d);
+  Expect.isTrue(d2 >= d);
+  Expect.isFalse(d2 < d);
+  Expect.isFalse(d2 <= d);
+  Expect.isFalse(d > d2);
+  Expect.isFalse(d >= d2);
+}