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

core/modules/serialization/tests/serialization_test/serialization_test.info.yml

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+name: Serialization test module
+type: module
+description: "Support module for serialization tests."
+package: Testing
+version: VERSION
+core: 8.x

core/modules/serialization/tests/serialization_test/serialization_test.services.yml

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+services:
+  serializer.normalizer.serialization_test:
+    class: Drupal\serialization_test\SerializationTestNormalizer
+    tags:
+      - { name: normalizer }
+  serializer.encoder.serialization_test:
+    class: Drupal\serialization_test\SerializationTestEncoder
+    tags:
+      - { name: encoder, format: serialization_test}

core/modules/serialization/tests/serialization_test/src/SerializationTestEncoder.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\serialization_test\SerializationTestEncoder.
+ */
+
+namespace Drupal\serialization_test;
+
+use Symfony\Component\Serializer\Encoder\EncoderInterface;
+
+class SerializationTestEncoder implements EncoderInterface {
+
+  /**
+   * The format that this Encoder supports.
+   *
+   * @var string
+   */
+  static protected $format = 'serialization_test';
+
+  /**
+   * Implements \Symfony\Component\Serializer\Encoder\EncoderInterface::encode().
+   */
+  public function encode($data, $format, array $context = array()) {
+    // @see \Drupal\serialization_test\SerializationTestNormalizer::normalize().
+    return 'Normalized by ' . $data['normalized_by'] . ', Encoded by SerializationTestEncoder';
+  }
+
+  /**
+   * Implements \Symfony\Component\Serializer\Encoder\EncoderInterface::supportsEncoding().
+   */
+  public function supportsEncoding($format) {
+    return static::$format === $format;
+  }
+}

core/modules/serialization/tests/serialization_test/src/SerializationTestNormalizer.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\serialization_test\SerializationTestNormalizer.
+ */
+
+namespace Drupal\serialization_test;
+
+use Symfony\Component\Serializer\Normalizer\NormalizerInterface;
+
+class SerializationTestNormalizer implements NormalizerInterface {
+
+  /**
+   * The format that this Normalizer supports.
+   *
+   * @var string
+   */
+  static protected $format = 'serialization_test';
+
+  /**
+   * Normalizes an object into a set of arrays/scalars.
+   *
+   * @param object $object
+   *   Object to normalize.
+   * @param string $format
+   *   Format the normalization result will be encoded as.
+   *
+   * @return array
+   *   An array containing a normalized representation of $object, appropriate
+   *   for encoding to the requested format.
+   */
+  public function normalize($object, $format = NULL, array $context = array()) {
+    $normalized = (array) $object;
+    // Add identifying value that can be used to verify that the expected
+    // normalizer was invoked.
+    $normalized['normalized_by'] = 'SerializationTestNormalizer';
+    return $normalized;
+  }
+
+  /**
+   * Checks whether format is supported by this normalizer.
+   *
+   * @param mixed  $data
+   *   Data to normalize.
+   * @param string $format
+   *   Format the normalization result will be encoded as.
+   *
+   * @return bool
+   *   Returns TRUE if the normalizer can handle the request.
+   */
+  public function supportsNormalization($data, $format = NULL) {
+    return static::$format === $format;
+  }
+}