article_zone_code_18.dart

abstract class Timer {
  factory Timer(Duration duration, void callback()) {
    // Create timer with current zone
    return Zone.current
        .createTimer(duration, Zone.current.bindCallbackGuarded(callback));
  }
  // ...
  // Create timer from environment
  external static Timer _createTimer(Duration duration, void callback());
}

class _RootZone implements Zone {
  // ...
  Timer createTimer(Duration duration, void f()) {
    return Timer._createTimer(duration, f);
  }
}