Issue #3165588 by longwave, mondrake: Add a check to ensure $modules property is always protected

core/tests/Drupal/Tests/Listeners/DrupalListener.php

@@ -96,6 +96,10 @@ class DrupalListener implements TestListener {
         }
       }
     }
+    // Check for incorrect visibility of the $modules property.
+    if ($class->hasProperty('modules') && !$class->getProperty('modules')->isProtected()) {
+      @trigger_error('The ' . get_class($test) . '::$modules property must be declared protected. See https://www.drupal.org/node/2909426', E_USER_DEPRECATED);
+    }
   }
 
   /**