Turns on a little "SLOW MODE" banner in checked mode to indicate that the app is in checked mode. This is on by default (in checked mode), to turn it off, set the constructor argument to false. In release mode this has no effect.
To get this banner in your application if you're not using WidgetsApp, include a CheckedModeBanner widget in your app.
This banner is intended to deter people from complaining that your app is slow when it's in checked mode. In checked mode, Flutter enables a large number of expensive diagnostics to aid in development, and so performance in checked mode is not representative of what will happen in release mode.
final bool debugShowCheckedModeBanner