RunHandler typedef Null safety
The type of a custom Zone.run implementation function.
Receives the Zone that the handler was registered on as
a delegate forwarding to the handlers of
self's parent zone as
and the current zone where the error was uncaught as
which will have
self as a parent zone.
f is the function which was passed to the
The default behavior of Zone.run is
f in the current zone,
A custom handler can do things before, after or instead of
typedef RunHandler = R Function<R>( Zone self, ZoneDelegate parent, Zone zone, R Function() f);