when<TResult extends Object?>  method 
- @optionalTypeArgs
A switch-like method, using callbacks.
As opposed to map, this offers destructuring.
It is equivalent to doing:
switch (sealedClass) {
  case Subclass(:final field):
    return ...;
  case Subclass2(:final field2):
    return ...;
}
Implementation
@optionalTypeArgs TResult when<TResult extends Object?>(TResult Function( String $type,  String? url,  Map<String, dynamic>? $unknown)  $default,) {final _that = this;
switch (_that) {
case _ServiceConfig():
return $default(_that.$type,_that.url,_that.$unknown);case _:
  throw StateError('Unexpected subclass');
}
}