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

core/modules/serialization/src/EntityResolver/EntityResolverInterface.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\serialization\EntityResolver\EntityResolverInterface.
+ */
+
+namespace Drupal\serialization\EntityResolver;
+
+use Symfony\Component\Serializer\Normalizer\NormalizerInterface;
+
+interface EntityResolverInterface {
+
+  /**
+   * Returns the local ID of an entity referenced by serialized data.
+   *
+   * Drupal entities are loaded by and internally referenced by a local ID.
+   * Because different websites can use the same local ID to refer to different
+   * entities (e.g., node "1" can be a different node on foo.com and bar.com, or
+   * on example.com and staging.example.com), it is generally unsuitable for use
+   * in hypermedia data exchanges. Instead, UUIDs, URIs, or other globally
+   * unique IDs are preferred.
+   *
+   * This function takes a $data array representing partially deserialized data
+   * for an entity reference, and resolves it to a local entity ID. For example,
+   * depending on the data specification being used, $data might contain a
+   * 'uuid' key, a 'uri' key, a 'href' key, or some other data identifying the
+   * entity, and it is up to the implementor of this interface to resolve that
+   * appropriately for the specification being used.
+   *
+   * @param \Symfony\Component\Serializer\Normalizer\NormalizerInterface $normalizer
+   *   The Normalizer which is handling the data.
+   * @param array $data
+   *   The data passed into the calling Normalizer.
+   * @param string $entity_type
+   *   The type of entity being resolved; e.g., 'node' or 'user'.
+   *
+   * @return string|null
+   *   Returns the local entity ID, if found. Otherwise, returns NULL.
+   */
+  public function resolve(NormalizerInterface $normalizer, $data, $entity_type);
+
+}