A clean and simple way to keep constants in Flutter is to make an own Dart library in the project for the constants.
const String BUTTON_LABEL = 'Submit';
The const keyword is used to represent a compile-time constant. Variables declared with const keyword are implicitly final. Link
You can then use import statement to access to the constants:
import 'package:<project_name>/assets/constants.dart' as constants; ... ElevatedButton( child: const Text(constants.BUTTON_LABEL), ) ...