$ah = Loader::helper('concrete/interface'); $ah->button_js(t('Add Image'), 'SlideshowBlock.chooseImg()', 'left')
This doesn't work anymore in 5.7. You get an error with the Loader::helper('concrete/interface') it says "Class helper/concrete/interface does not exist". Then with just the button_js line the form does not load at all.
Is that interface helper gone in 5.7 or what needs to be done to make it work?
$ah = Loader::helper('concrete/ui');
$ah = Core::make('helper/concrete/ui');
That said, the Loader class is still used in various places throughout the core, so I guess there's no big rush to adopt the new approach, but it should make your code a bit more future-proof.