Rename DateTime constants to full names.

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 b83e96bb77759de62a6ffa9efeac9ff1de100d22..c1336c47badd131f9f0015054d49f5ecc7719dcb 100644
--- a/runtime/lib/date_patch.dart
+++ b/runtime/lib/date_patch.dart
@@ -75,8 +75,9 @@ patch class DateTime {
     int daysSince1970 =
         _flooredDivision(_localDateInUtcMs, Duration.MILLISECONDS_PER_DAY);
     // 1970-1-1 was a Thursday.
-    return ((daysSince1970 + DateTime.THU - DateTime.MON) % DateTime.DAYS_IN_WEEK) +
-        DateTime.MON;
+    return ((daysSince1970 + DateTime.THURSDAY - DateTime.MONDAY)
+            % DateTime.DAYS_IN_WEEK) +
+        DateTime.MONDAY;
   }