I want to use named parameters in Dart for clarity. How should I handle them?

The meta package provides a @required annotation that is supported by the DartAnalyzer.

Flutter uses this a lot and provides @required directly from import 'package:flutter/foundation.dart'

foo({@required String name}) {...}

foo(); // results in static warning

@required doesn’t check if the passed value is null or not, only that a value was actually passed on the call site.
To check for null you can also use assert() to check for passed values

class Ability {
  Ability(this.name, this.effectDuration, this.recast) : assert(name != null), assert(effectDuration != null), assert(recast != null);
  final name;
  final effectDuration;
  final recast;            // wait time until next use
  // ...
}