Unify Date.fromString and support more Iso 8601.

frog/leg/lib/date_helper.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.
 
+String fourDigits(int n) {
+  int absN = n.abs();
+  String sign = n < 0 ? "-" : "";
+  if (absN >= 1000) return "$n";
+  if (absN >= 100) return "${sign}0$absN";
+  if (absN >= 10) return "${sign}00$absN";
+  if (absN >= 1) return "${sign}000$absN";
+}
+
 String threeDigits(int n) {
   if (n >= 100) return "${n}";
   if (n > 10) return "0${n}";
   return "00${n}";
 }
 
 String twoDigits(int n) {
   if (n >= 10) return "${n}";
   return "0${n}";
 }