// config/constants/app_constants.dart
class AppConstants {
  static const baseUrl = String.fromEnvironment(
    'BASE_URL',
    defaultValue: 'https://dev-api.example.com',
  );
}

# Single value
flutter run --dart-define=BASE_URL=https://api.example.com

# Full environment file (dev / staging / prod)
flutter run --dart-define-from-file=env/prod.json

// env/prod.json
{
  "BASE_URL": "https://api.example.com",
  "ENV":      "production"
}