Some previous work and exploration were done in this area such as flutter/engine#20433, flutter/engine#18047 and flutter/engine#20142.

1- Split out a path to asynchronously create the VM in the FlutterEngineGroup ahead of engine creation.

@mkustermann says "Separate Dart_Initialize() (once per process) from Dart_CreateIsolateGroup*() (once per isolate-group, reads AOT snapshot) and Dart_CreateIsolateInGroup() (once per lightweight UI isolate)."

2- Make the new APIs #72010 and #72018 work asynchronously to not block the main thread when creating new engines and isolates, such as when creating Flutter views in a scrollable list.