apiServerURL is unable to updated from environment.

[joyhope]

The code const is wrong.
var url = const String.fromEnvironment( 'API_SERVER_URL', defaultValue: defaultAPIServerURL, );

With the const, the URL is always defaultAPISserverURL, the const should be removed if want to support ENV parameter setup.

[joyhope]

otherwise, the current only supports compile-time define, with --dart-define.

If supports the environment setup, it is easy to use the released version. It is easy for user to switch for the different background servers.