* [dev] Don't use await for on stdout and stdin; pass local engine argument

1) Don't use await for on stdout followd by await for on stderr.
   This can cause nothing to happen. E.g. if one didn't do something
   like `flutter pub upgrade` dev/automated_tests and the version in the
   old one was pre-nnbd (but flutter itself use lots of nnbd stuff) lots
   of errors would be emitted, but because we basically only listen to
   stdout nothing will happen (deadlock once stderr buffer runs out).
   Note that it still awaits the exit-code below.
2) Pass --local-engine (if given) so there won't be any crashes because
   of that.