Update to flutter-2.8.0 (#68)

Updated to the flutter 2.8.0. See also:
 - https://github.com/sony/flutter-embedded-linux/releases/tag/40a99c5951
 - https://github.com/flutter/flutter/tree/2.8.0

lib/executable.dart

@@ -141,7 +141,6 @@ Future<void> main(List<String> args) async {
               stdio: globals.stdio,
               terminal: globals.terminal,
               outputPreferences: globals.outputPreferences,
-              stopwatchFactory: const StopwatchFactory(),
             )),
     },
   );