Update corelib to revised factory syntax.

corelib/src/time_zone.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Dart core library.
 
 /**
  * [TimeZone]s represent locations (for example Europe/Paris).
  */
-interface TimeZone factory TimeZoneImplementation {
+interface TimeZone default TimeZoneImplementation {
   const TimeZone.utc();
   TimeZone.local();
   bool get isUtc();
 }