providerToLink property
Links this user to the specified provider.
Linking a provider to an existing user account does not invalidate the refresh token of that account. In other words, the existing account would continue to be able to access resources, despite not having used the newly linked provider to log in. If you wish to force the user to authenticate with this new provider, you need to (a) revoke their refresh token (see https://firebase.google.com/docs/auth/admin/manage-sessions#revoke_refresh_tokens), and (b) ensure no other authentication methods are present on this account.
Implementation
final UserProvider? providerToLink;