The refactoring preview shows the list of changes.
Finally, click Do Refactor:

That is it! You successfully migrated your project to AndroidX.

Finally, if you migrated a plugin,
publish the new AndroidX version to pub and update
your CHANGELOG.md to indicate that this new version
is compatible with AndroidX.

What if I can’t use Android Studio?

You can create a new project using the Flutter tool
and then move the Dart code and
assets to the new project.

To create a new project run:

t <project-type> <new-project-path>

Add to app

If your Flutter project is a module type for adding
to an existing Android app, and contains a
.android directory, add the following line to pubspec.yaml:

module

...

androidX

true

# Add this line.