migrate raw string to new syntax in core

lib/coreimpl/date.dart

Index: lib/coreimpl/date.dart
===================================================================
--- lib/coreimpl/date.dart	(revision 12701)
+++ lib/coreimpl/date.dart	(working copy)
@@ -17,8 +17,8 @@
     //    - "2012-02-27T14Z"
     //    - "-123450101 00:00:00 Z"  // In the year -12345.
     final RegExp re = const RegExp(
-        @'^([+-]?\d?\d\d\d\d)-?(\d\d)-?(\d\d)'  // The day part.
-        @'(?:[ T](\d\d)(?::?(\d\d)(?::?(\d\d)(.\d{1,6})?)?)? ?([zZ])?)?$');
+        r'^([+-]?\d?\d\d\d\d)-?(\d\d)-?(\d\d)'  // The day part.
+        r'(?:[ T](\d\d)(?::?(\d\d)(?::?(\d\d)(.\d{1,6})?)?)? ?([zZ])?)?$');
     Match match = re.firstMatch(formattedString);
     if (match !== null) {
       int parseIntOrZero(String matched) {