mapOrElse<U>  method 
      
U
mapOrElse<U>( 
    
- U defaultFn(),
 - U f(
- T
 
 
Computes a default function result (if none), or applies a different function to the contained value (if any).
Implementation
U mapOrElse<U>(U Function() defaultFn, U Function(T) f) {
  if (v == null) {
    return defaultFn();
  } else {
    return f(v!);
  }
}