Make most arguments to Date constructor optional.

frog/lib/date_implementation.dart

Index: frog/lib/date_implementation.dart
diff --git a/frog/lib/date_implementation.dart b/frog/lib/date_implementation.dart
index 70747d573f7b0628efef723f583a11abca1206f3..2fea939f15a772e65c0cfce5b9482cd0a3601aa1 100644
--- a/frog/lib/date_implementation.dart
+++ b/frog/lib/date_implementation.dart
@@ -9,12 +9,12 @@ class DateImplementation implements Date {
   final TimeZone timeZone;
 
   factory DateImplementation(int years,
-                             int month,
-                             int day,
-                             int hours,
-                             int minutes,
-                             int seconds,
-                             int milliseconds) {
+                             [int month = 1,
+                              int day = 1,
+                              int hours = 0,
+                              int minutes = 0,
+                              int seconds = 0,
+                              int milliseconds = 0]) {
     return new DateImplementation.withTimeZone(
         years, month, day,
         hours, minutes, seconds, milliseconds,