Problem/Motivation
$row_classes should be on the context of the row which is being rendered.
Proposed resolution
Move $row_classes to $row['attributes'] with the Attribute class.
This will need to move to move the $row nested further to $row['content']
User interface changes
n/a
API changes
$row_classes will no longer be available in views templates. Need to be converted to print $row['attributes'], and in twig {{ row.attributes }}
and $row will become $row['content'] or {{ row.attributes }}
Related Issues
#1843754: Convert views/templates/views-view-list.tpl.php to twig
#1843770: Convert views/templates/views-view-unformatted.tpl.php to twig