Add an AuthenticationChallenge class.

lib/src/http_date.dart

 // Copyright (c) 2014, 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.
 
 library http_parser.http_date;
 
 import 'package:string_scanner/string_scanner.dart';
 
+import 'utils.dart';
+
 const _WEEKDAYS = const ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"];
 const _MONTHS = const ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug",
     "Sep", "Oct", "Nov", "Dec"];
 
 final _shortWeekdayRegExp = new RegExp(r"Mon|Tue|Wed|Thu|Fri|Sat|Sun");
 final _longWeekdayRegExp =
     new RegExp(r"Monday|Tuesday|Wednesday|Thursday|Friday|Saturday|Sunday");
 final _monthRegExp =
     new RegExp(r"Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec");
 final _digitRegExp = new RegExp(r"\d+");
@@ skipping 22 matching lines @@
     ..write(" GMT");
   return buffer.toString();
 }
 
 /// Parses an HTTP-formatted date into a UTC [DateTime].
 ///
 /// This follows [RFC
 /// 2616](http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.3). It will
 /// throw a [FormatException] if [date] is invalid.
 DateTime parseHttpDate(String date) {
-  try {
+  return wrapFormatException("HTTP date", date, () {
     var scanner = new StringScanner(date);
 
     if (scanner.scan(_longWeekdayRegExp)) {
       // RFC 850 starts with a long weekday.
       scanner.expect(", ");
       var day = _parseInt(scanner, 2);
       scanner.expect("-");
       var month = _parseMonth(scanner);
       scanner.expect("-");
       var year = 1900 + _parseInt(scanner, 2);
@@ skipping 27 matching lines @@
     var month = _parseMonth(scanner);
     scanner.expect(" ");
     var day = scanner.scan(" ") ? _parseInt(scanner, 1) : _parseInt(scanner, 2);
     scanner.expect(" ");
     var time = _parseTime(scanner);
     scanner.expect(" ");
     var year = _parseInt(scanner, 4);
     scanner.expectDone();
 
     return _makeDateTime(year, month, day, time);
-  } on FormatException catch (error) {
-    throw new FormatException('Invalid HTTP date "$date": ${error.message}');
-  }
+  });
 }
 
 /// Parses a short-form month name to a form accepted by [DateTime].
 int _parseMonth(StringScanner scanner) {
   scanner.expect(_monthRegExp);
   // DateTime uses 1-indexed months.
   return _MONTHS.indexOf(scanner.lastMatch[0]) + 1;
 }
 
 /// Parses an int an enforces that it has exactly [digits] digits.
@@ skipping 29 matching lines @@
 DateTime _makeDateTime(int year, int month, int day, DateTime time) {
   var dateTime =
       new DateTime.utc(year, month, day, time.hour, time.minute, time.second);
 
   // If [day] was too large, it will cause [month] to overflow.
   if (dateTime.month != month) {
     throw new FormatException("invalid day '$day' for month '$month'.");
   }
   return dateTime;
 }