This shell used to serve the dual purpose of running Flutter tests as
well as optionally being able to run Flutter applications on a Mac
desktop. After all the test runner targets were consolidated
(flutter_tester), this target was no longer used to run the tests. This
target is currently only used by engine developers to test changes
quickly on the host. Even for this purpose, the Desktop embedders are
more full featured and use the stable embedder API. The presence of a
cross-platform tester target, alternatives for Desktop targets that use
the stable embedder API and the fact that this target is no longer
tested, makes having this code in the source entirely redundant.