Issue #2809801 by ziomizar: provider property is dynamically declared in UrlGenerator

core/lib/Drupal/Core/Routing/UrlGenerator.php

@@ -19,6 +19,13 @@ use Symfony\Component\Routing\Exception\MissingMandatoryParametersException;
  */
 class UrlGenerator implements UrlGeneratorInterface {
 
+  /**
+   * The route provider.
+   *
+   * @var \Drupal\Core\Routing\RouteProviderInterface
+   */
+  protected $provider;
+
   /**
    * @var RequestContext
    */