ZoneDelegate says:

When forwarding to parent zones one can't just invoke the parent zone's exposed functions (like Zone.run), but one needs to provide more information (like the zone the run was initiated). Zone callbacks thus receive more information including this ZoneDelegate class. When delegating to the parent zone one should go through the given instance instead of directly invoking the parent zone.

...but that just raises more questions. Why do you need to provide more information? What is "the zone the run was initiated"?

Further, the individual functions on this class have no documentation at all. If you're in a handleUncaughtHandler and you want to run something in the context of the parent zone, what are the arguments you pass to parent.run, for example?