__experimentalMainDashboardButton slot no longer replace existing button with 18.5.0-rc1.

[yansern]

Description

From the MainDashboardButton documentation, slots can be used to replace the existing dashboard button.

However, I suspect with the change here, now both third-party slots and the original button from core are displaying next to each other.

Step-by-step reproduction instructions

You replicate this issue by following the documentation here on button replacement.

Calypso uses this slot as seen here so by switching testing your site on Gutenberg Edge you should see the issue.

Screenshots, screen recording, code snippet

Environment info

• Gutenberg 18.5.0-rc1.
• Tested on Gutenberg Edge on WPCOM (but can be reproduced locally using the documentation here).

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

No