Tried MySQL 5.6.5, 5.6.10, 5.6.12, errors are different, but the result is the same. The example below is for MySQL 5.6.10.
create a new datadir (system tables) in MySQL 5.6;
try to start MariaDB 10.0 on this datadir
=> error log shows errors:
Run mysql_upgrade, it throws errors and fails
The event table is still broken: