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

vendor/symfony/serializer/NameConverter/CamelCaseToSnakeCaseNameConverter.php

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+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Serializer\NameConverter;
+
+/**
+ * CamelCase to Underscore name converter.
+ *
+ * @author Kévin Dunglas <dunglas@gmail.com>
+ */
+class CamelCaseToSnakeCaseNameConverter implements NameConverterInterface
+{
+    /**
+     * @var array|null
+     */
+    private $attributes;
+    /**
+     * @var bool
+     */
+    private $lowerCamelCase;
+
+    /**
+     * @param null|array $attributes     The list of attributes to rename or null for all attributes.
+     * @param bool       $lowerCamelCase Use lowerCamelCase style.
+     */
+    public function __construct(array $attributes = null, $lowerCamelCase = true)
+    {
+        $this->attributes = $attributes;
+        $this->lowerCamelCase = $lowerCamelCase;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function normalize($propertyName)
+    {
+        if (null === $this->attributes || in_array($propertyName, $this->attributes)) {
+            $snakeCasedName = '';
+
+            $len = strlen($propertyName);
+            for ($i = 0; $i < $len; ++$i) {
+                if (ctype_upper($propertyName[$i])) {
+                    $snakeCasedName .= '_'.strtolower($propertyName[$i]);
+                } else {
+                    $snakeCasedName .= strtolower($propertyName[$i]);
+                }
+            }
+
+            return $snakeCasedName;
+        }
+
+        return $propertyName;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function denormalize($propertyName)
+    {
+        $camelCasedName = preg_replace_callback('/(^|_|\.)+(.)/', function ($match) {
+            return ('.' === $match[1] ? '_' : '').strtoupper($match[2]);
+        }, $propertyName);
+
+        if ($this->lowerCamelCase) {
+            $camelCasedName = lcfirst($camelCasedName);
+        }
+
+        if (null === $this->attributes || in_array($camelCasedName, $this->attributes)) {
+            return $this->lowerCamelCase ? lcfirst($camelCasedName) : $camelCasedName;
+        }
+
+        return $propertyName;
+    }
+}

vendor/symfony/serializer/NameConverter/NameConverterInterface.php

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+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Serializer\NameConverter;
+
+/**
+ * Defines the interface for property name converters.
+ *
+ * @author Kévin Dunglas <dunglas@gmail.com>
+ */
+interface NameConverterInterface
+{
+    /**
+     * Converts a property name to its normalized value.
+     *
+     * @param string $propertyName
+     *
+     * @return string
+     */
+    public function normalize($propertyName);
+
+    /**
+     * Converts a property name to its denormalized value.
+     *
+     * @param string $propertyName
+     *
+     * @return string
+     */
+    public function denormalize($propertyName);
+}