Update to Drupal 8.2.0. For more information, see https://www.drupal.org/project/drupal/releases/8.2.0

core/lib/Drupal/Core/Serialization/Yaml.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace Drupal\Core\Serialization;
+
+use Drupal\Core\Site\Settings;
+use Drupal\Component\Serialization\Yaml as ComponentYaml;
+
+/**
+ * Provides a YAML serialization implementation.
+ *
+ * Allow settings to override the YAML implementation resolution.
+ */
+class Yaml extends ComponentYaml {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static function getSerializer() {
+    // Allow settings.php to override the YAML serializer.
+    if (!isset(static::$serializer) &&
+      $class = Settings::get('yaml_parser_class')) {
+
+      static::$serializer = $class;
+    }
+    return parent::getSerializer();
+  }
+
+}