setup method
void
setup()
Implementation
void setup() {
if ( typeDescriptor.findAnnotation<JsonSerializable>()?.discriminator != null)
mappings[typeDescriptor.findAnnotation<JsonSerializable>()!.discriminator] = this;
for (var subClass in typeDescriptor.childClasses) {
var discriminator = subClass.findAnnotation<JsonSerializable>()!.discriminator;
mappings[discriminator] = mapper.getMappingX(Map<String, dynamic>, subClass.type);
}
}