This commit enhances the `CompileFn`, which is used to process each
entry-point, to support running a passed-in callback (and wait for it to
complete) before proceeding with writing the transformed files to disk.
This functionality is currently not used. In a subsequent commit, it
will be used for passing info from worker processes to the master
process that will allow ngcc to recover when a worker process crashes in
the middle of processing a task.
PR Close#36626
George Kalpakas
e367593a26