[frontend_server] Migrate frontend_server_starter

Change-Id: Ic68ba05b920c18cc6d13c5141b71f6c4cb6eca84
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/259580
Reviewed-by: Jens Johansen <jensj@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

pkg/frontend_server/bin/frontend_server_starter.dart

@@ -1,4 +1,7 @@
-// @dart = 2.9
+// Copyright (c) 2018, 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.
+
 library frontend_server;
 
 import 'dart:async';