CreatePeriodicTimerHandler typedef Null safety

The type of a custom Zone.createPeriodicTimer implementation function.

Receives the Zone that the handler was registered on as self, a delegate forwarding to the handlers of self's parent zone as parent, and the current zone where the error was uncaught as zone, which will have self as a parent zone.

The callback function f and period are the ones which were passed to Zone.createPeriodicTimer of zone (possibly through the Timer.periodic constructor).

The custom handler can choose to replace the function f with one that does something before, after or instead of calling f, and then call parent.createTimer(zone, replacement). or it can implement its own timer queue, which typically still depends on parent.createTimer to as a way to get started.

The function should return a Timer object which can be used to inspect and control the scheduled timer callbacks.