The root zone.
All isolate entry functions (
main or spawned functions) start running in
the root zone (that is, Zone.current is identical to Zone.ROOT when the
entry function is called). If no custom zone is created, the rest of the
program always runs in the root zone.
The root zone implements the default behavior of all zone operations. Many methods, like registerCallback do the bare minimum required of the function, and are only provided as a hook for custom zones. Others, like scheduleMicrotask, interact with the underlying system to implement the desired behavior.