After upgrading HHVM to version 3.10 from 3.9.1 and rebuilding luasandbox (sudo hphpize && cmake . && sudo make install && sudo service hhvm restart) luasandox is enabled: phpinfo () shows:
hhvm.dynamic_extensions = Array (  => luasandbox.so ) ... hhvm.enable_zend_compat = 1
However, luasandbox doesn't work, the class Luasandbox does not exist and /var/log/hhvm/error.log says Fatal error: Class undefined: LuaSandbox in (path to MediaWiki)/extensions/Scribunto/engines/LuaSandbox/Engine.php on line 17.
Upgrading luasandbox didn't help.
If I run hhvm luasandbox.php this is what I get:
Fatal error: f_get_extension_funcs is not supported: extensions are built differently in (path to MediaWiki)/extensions/luasandbox/luasandbox.php on line 8
Upgrade: I downgraded HHVM to 3.9.1 and then to 3.9 but the issue remained. I suspect now, that the error is in LuaSandbox.
Upgrade: under HHVM 3.10.1 the issue persists.