Change Duration to take proper named optional parameters.

lib/coreimpl/duration_implementation.dart

 // Copyright (c) 2011, 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.
 
 // Dart core library.
 
 class DurationImplementation implements Duration {
   final int inMilliseconds;
 
-  const DurationImplementation([int days = 0,
-                                int hours = 0,
-                                int minutes = 0,
-                                int seconds = 0,
-                                int milliseconds = 0])
+  const DurationImplementation({int days: 0,
+                                int hours: 0,
+                                int minutes: 0,
+                                int seconds: 0,
+                                int milliseconds: 0})
     : inMilliseconds = days * Duration.MILLISECONDS_PER_DAY +
                        hours * Duration.MILLISECONDS_PER_HOUR +
                        minutes * Duration.MILLISECONDS_PER_MINUTE +
                        seconds * Duration.MILLISECONDS_PER_SECOND +
                        milliseconds;
 
   int get inDays {
     return inMilliseconds ~/ Duration.MILLISECONDS_PER_DAY;
   }
 
@@ skipping 40 matching lines @@
     }
     String twoDigitMinutes =
         twoDigits(inMinutes.remainder(Duration.MINUTES_PER_HOUR));
     String twoDigitSeconds =
         twoDigits(inSeconds.remainder(Duration.SECONDS_PER_MINUTE));
     String threeDigitMs =
         threeDigits(inMilliseconds.remainder(Duration.MILLISECONDS_PER_SECOND));
     return "${inHours}:${twoDigitMinutes}:${twoDigitSeconds}.${threeDigitMs}";
   }
 }