class AppConstants {
  static const baseUrl = String.fromEnvironment(
    'BASE_URL',
    defaultValue: 'https://api.example.com',
  );

  static const env = String.fromEnvironment(
    'ENV',
    defaultValue: 'development',
  );
}

# 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

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