Removed Type-Defect label.
Added Type-Enhancement label.

Hi. Will it be implemented? It would be very convenient.

I'm wary about adding more overhead to zone entry/exit (it will cost, at least one lookup of "is onZoneEnterOrExit overridden").

At a fundamental level, a zone isn't being "enterede" and "exited", it's just that the currently executing code has a zone associated with it. That zone can be changed in a number of ways, mostly by Zone.run, but internally (e.g., in the microtask queue) it's also being done just by selecting the zone and running the code.

Entering a zone is usually somewhat well-delimited, but exiting is harder to define. Are you exiting a zone when you enter another zone with zone.run? Are you reentering the original zone when that call returns? Does it matter whether the new zone is/is not a child of the original zone? Maybe you should only call onExit when you switch to a zone that has a different onEnter/onExit implementation registered, not one that inherits the current one (like we do for error handlers). But that might add even more overhead in the checks, or maybe it's just one identical check on the _onEnterExitHandler property.

In any case, it's not without cost, even if you don't use it. There is no such thing as a free lunch :)