Adding this in the widget form method:
wp_editor($text, $this->get_field_id('text'));
seems to work. But after you press the "Save" button the widget goes funky...
Adding _WP_Editors::editor_js();
makes it show after you press the save button, but then other things start to act weird. I think it has something to do with the javascript from the footer.
Has anyone successfully integrated wp_editor inside a widget?