Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/lib/Drupal/Core/ProxyBuilder/ProxyBuilder.php

@@ -0,0 +1,28 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\ProxyBuilder\ProxyBuilder.
+ */
+
+namespace Drupal\Core\ProxyBuilder;
+
+use Drupal\Component\ProxyBuilder\ProxyBuilder as BaseProxyBuilder;
+
+/**
+ * Extend the component proxy builder by using the DependencySerialziationTrait.
+ */
+class ProxyBuilder extends BaseProxyBuilder {
+
+  /**
+   * {@inheritdoc{
+   */
+  protected function buildUseStatements() {
+    $output = parent::buildUseStatements();
+
+    $output .= 'use \Drupal\Core\DependencyInjection\DependencySerializationTrait;' . "\n\n";
+
+    return $output;
+  }
+
+}