Here is the warning text:

Notice: Undefined property: Drupal\Core\Language\Language::$language in accessibility_accessibility_get_test_translation() (line 330 of /Users/jbeach/code/drupal/modules/d8/accessibility/accessibility.module).
Warning: trim() expects parameter 1 to be string, array given in Drupal\Component\Gettext\PoStreamReader->readHeader() (line 239 of core/lib/Drupal/Component/Gettext/PoStreamReader.php).

This happens on /admin/config/accessibility/tests/import

I'm guessing something about how translation is implemented changed in Drupal 8.