PrintHandler typedef Null safety
The type of a custom Zone.print 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.
The custom handler can intercept print operations and redirect them to other targets than the console.
typedef void PrintHandler( Zone self, ZoneDelegate parent, Zone zone, String line);