Crear la clase de excepción en flutter

dart excepción

try {
  breedMoreLlamas();
} on OutOfLlamasException {			// A specific exception  
  buyMoreLlamas();
} on Exception catch (e) { 			// Anything else that is an exception
  print('Unknown exception: $e');
} catch (e) {						// No specified type, handles all
  print('Something really unknown: $e');
} finally {							// Always clean up, even if case of exception
  cleanLlamaStalls();
}