Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

core/lib/Drupal/Core/Ajax/BaseCommand.php

@@ -0,0 +1,52 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Ajax\BaseCommand.
+ */
+
+namespace Drupal\Core\Ajax;
+
+/**
+ * Base command that only exists to simplify AJAX commands.
+ */
+class BaseCommand implements CommandInterface {
+
+  /**
+   * The name of the command.
+   *
+   * @var string
+   */
+  protected $command;
+
+  /**
+   * The data to pass on to the client side.
+   *
+   * @var string
+   */
+  protected $data;
+
+  /**
+   * Constructs a BaseCommand object.
+   *
+   * @param string $command
+   *   The name of the command.
+   * @param string $data
+   *   The data to pass on to the client side.
+   */
+  public function __construct($command, $data) {
+    $this->command = $command;
+    $this->data = $data;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function render() {
+    return array(
+      'command' => $this->command,
+      'data' => $this->data,
+    );
+  }
+
+}

core/lib/Drupal/Core/Ajax/CommandWithAttachedAssetsTrait.php

@@ -29,7 +29,7 @@ trait CommandWithAttachedAssetsTrait {
    * If content is a render array, it may contain attached assets to be
    * processed.
    *
-   * @return string|\Drupal\Component\Utility\SafeStringInterface
+   * @return string|\Drupal\Component\Render\MarkupInterface
    *   HTML rendered content.
    */
   protected function getRenderedContent() {