Change Duration to take proper named optional parameters.

lib/coreimpl/duration_implementation.dart

Index: lib/coreimpl/duration_implementation.dart
diff --git a/lib/coreimpl/duration_implementation.dart b/lib/coreimpl/duration_implementation.dart
index d93043647ddfd3d3764d12d2a542da4becacee7e..9ea8e360b066238baf281fdecd22d7d502fa01d9 100644
--- a/lib/coreimpl/duration_implementation.dart
+++ b/lib/coreimpl/duration_implementation.dart
@@ -7,11 +7,11 @@
 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 +