map1<R, T1>  static method 
Maps the evaluation of the 1-argument callback to an event.
Implementation
// ignore: prefer_constructors_over_static_methods
static Event<R> map1<R, T1>(Map1<T1, R> callback, T1 value1) {
  try {
    return Event<R>.next(callback(value1));
  } catch (error, stackTrace) {
    return Event<R>.error(error, stackTrace);
  }
}