8.6.7 to .8 upgrade breaks site: Class 'Drupal\locale\SourceString' not found

Running drush up:

/.../
Code updates will be made to drupal core.
WARNING: /.../
Note: /.../

Do you really want to continue? (y/n): y
Drush command terminated abnormally due to an unrecoverable error. [error]
Error: Class 'Drupal\locale\SourceString' not found in
core/modules/locale/src/StringDatabaseStorage.php, line 226
[~/www/hrana]# drush up drupal
Command pm-update needs a higher bootstrap level to run - you will need to invoke drush from a more [error]
functional Drupal environment to run this command.
The drush command 'up drupal' could not be executed. [error]

So the site and drush got crippled — the core, vendor and libraries dirs are now missing. And all toplevel files; somehow a few folders survived.

Drush Version : 8.1.13

There was only one module update before that and it finished with a success:

Admin Toolbar (admin_toolbar) 8.x-1.25 8.x-1.26 Update available