Rename DAYS_IN_WEEK to DAYS_PER_WEEK. Add MONTHS_PER_YEAR.

runtime/lib/date_patch.dart

Index: runtime/lib/date_patch.dart
diff --git a/runtime/lib/date_patch.dart b/runtime/lib/date_patch.dart
index c1336c47badd131f9f0015054d49f5ecc7719dcb..b1f2f8dde6cdaee1c505c5301af6a979ca40ae36 100644
--- a/runtime/lib/date_patch.dart
+++ b/runtime/lib/date_patch.dart
@@ -76,7 +76,7 @@ patch class DateTime {
         _flooredDivision(_localDateInUtcMs, Duration.MILLISECONDS_PER_DAY);
     // 1970-1-1 was a Thursday.
     return ((daysSince1970 + DateTime.THURSDAY - DateTime.MONDAY)
-            % DateTime.DAYS_IN_WEEK) +
+            % DateTime.DAYS_PER_WEEK) +
         DateTime.MONDAY;
   }