diff --git a/core/lib/Drupal/Core/DependencyInjection/ContainerBuilder.php b/core/lib/Drupal/Core/DependencyInjection/ContainerBuilder.php
index 00b6f761ae..7c55191e86 100644
--- a/core/lib/Drupal/Core/DependencyInjection/ContainerBuilder.php
+++ b/core/lib/Drupal/Core/DependencyInjection/ContainerBuilder.php
@@ -104,6 +104,19 @@ public function setAlias($alias, $id) {
     return $alias;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function setDefinition($id, Definition $definition) {
+    $definition = parent::setDefinition($id, $definition);
+    // As of Symfony 3.4 all definitions are private by default. Only adjust
+    // them if the definition has not been marked non-public.
+    if ($definition->isPublic()) {
+      $definition->setPrivate(FALSE);
+    }
+    return $definition;
+  }
+
   /**
    * {@inheritdoc}
    */