had same issue, was going to add the define(‘CONCATENATE_SCRIPTS’, false) to wp-config.php but when opened that file I noticed my wp-config.php had the closing ?> at the end of the document. Noticed there is no closing ?> in wp-config-sample.php in 3.5. So before trying the above fix I just removed the ?> from my wp-config.php file and the visual/html tabs are back.
This site I upgrade is pretty old so that closing ?> probably was a remnant from a 2.x version
hope this helps somebody
