Except the abstract Base class in Fetchers folder, all other files were removed since version 1.2.0,
but Runner.php still contain a line that try to instantiate a new object of type \WP_CLI\Fetchers\User.

The fatal error happens if the command line is invoked with --user='any-name' and the constant WP_INSTALLING has not been defined.

Note: WP_INSTALLING is a constant which gets defined by Wordpress when it start installation for a fresh site.