1.15 Breaks on Pantheon

The 1.15 update seems to break on sites where the filesystem isn’t writable, namely Pantheon “Test” and “Live” environments (and “Dev” when the connection method is set to “Git”).

It definitely seems related to the writability of the filesystem, because in a local environment that is normally writable, when I change the permissions so that the filesystem is _not_ writeable by the web server process, I get this stack trace:

PHP Fatal error:  Uncaught Error: Class 'OTW_Shortcode_Widget' not found in /var/www/educatefairfax-site/web/wp/wp-includes/class-wp-widget-factory.php:61
Stack trace:
#0 /var/www/educatefairfax-site/web/wp/wp-includes/widgets.php(115): WP_Widget_Factory->register()
#1 /var/www/educatefairfax-site/web/wp-content/plugins/buttons-shortcode-and-widget/include/otw_bsw_functions.php(26): register_widget()
#2 /var/www/educatefairfax-site/web/wp/wp-includes/class-wp-hook.php(303): otw_bsw_widgets_init()
#3 /var/www/educatefairfax-site/web/wp/wp-includes/class-wp-hook.php(327): WP_Hook->apply_filters()
#4 /var/www/educatefairfax-site/web/wp/wp-includes/plugin.php(470): WP_Hook->do_action()
#5 /var/www/educatefairfax-site/web/wp/wp-includes/widgets.php(1809): do_action()
#6 /var/www/educatefairfax-site/web/wp/wp-includes/class-wp-hook.php(303): wp_widgets_init()
#7 /var/www/educatefairfax-site/web/wp/wp-includes/class-wp-hook.php(327): WP_Hook->apply_filters()
#8 /var/www/educatefairfax-site/web/wp/wp-includes/plugin.php(470): WP_Hook- in /var/www/educatefairfax-site/web/wp/wp-includes/class-wp-widget-factory.php on line 61

(And yes, the “uploads” directory _is_ still writable.)

The resulting screen I get on the front end is here: https://www.dropbox.com/s/bnvexng1fp4ps3u/button-shortcode-widget.png?dl=0

Any help on this is appreciated, thanks.

— Tracy