I had this same problem with the visual editor not showing up.
I noticed some errors in the javascript console. It looks like something was renamed in reference to tinymce (the visual editor code).
I had to rename
wp-includes/js/tinymce/langs/wp-langs-en.js
to
wp-includes/js/tinymce/langs/en.js
and also copy the “en.js” file here:
wp-includes/js/tinymce/advanced/langs/en.js
Then the editor works again.