VM calls embedder supplied isolate group cleanup callback when the vm-isolate shuts down

[dnfield]

Take the test from flutter/engine#48478, or really any of the tests in that file that create an isolate group.

Add the --trace-shutdown flag to the VM

Add a log to DartIsolate::DartIsolateGroupCleanupCallback or observe the timeline.

Observe that DartIsolate::DartIsolateGroupCleanupCallback is called twice, once for the group that's created shutting down (main) and a second time for the vm-isolate. On the second call the isolate_data parameter is nullptr.

This is harmless for Flutter right now but a bit confusing, and could lead to problems if we added more code to our cleanup callback.

/cc @mkustermann fyi since we talked about this in chat.