Hi @ondrejmirtes, I just got an issue with my tests saying that an offset does not exist in an array.
I tried to add a assertArrayHasKey check, without success.

I think this assert is not handle by this plugin, am I wrong ?

I'll try to do a PR in order to add the support, I think I'll be able to copy the way isArray works.
https://github.com/phpstan/phpstan-phpunit/blob/master/src/Type/PHPUnit/Assert/AssertTypeSpecifyingExtensionHelper.php#L158