How to discern if an optional named parameter in dart was given?

You can use a special marker in the file like this

const _noValueGiven = const Object();

class State {
  ...
  State copy({String firstName: _noValueGiven, String lastName: _noValueGiven}) {
    if (identical(firstName, _noValueGiven)) {
      firstName = this.firstName;
    }
    ...
  }
}