Symfony version(s) affected: 4.2.0

Description
The SonataAdminBundle build breaks because we have form types that suddenly need to implement getExtendedTypes(). This lines seems to assume the method exists when it actually does not:

How to reproduce

git clone git@github.com:sonata-project/SonataAdminBundle.git
cd SonataAdminBundle
git checkout 437ed2a9aa09fac6bef66abeaf1e5eeb9b32796d
composer install
wget -O phpunit https://phar.phpunit.de/phpunit-5.phar
chmod +x phpunit
./phpunit tests/Form/Type/AdminTypeTest.php

Possible Solution
Provide a default implementation for that method, that would check if getExtendedType() is overriden in the child class, and trigger a deprecation if it is the case, call it and wrap the result in an array.

That cannot work since the new method has to be static…