Programmatically embed an entity edit form in Drupal 8

node_edit_form_d8.php

<?php

// Without the use of D.I.
$form = \Drupal::service('entity.manager')
  ->getFormObject('{ENTITY_TYPE}', 'default')
  ->setEntity($entity);
$build[] = \Drupal::formBuilder()->getForm($form);

// Within a class using D.I.
class someClass {

  /**
   * The entity manager.
   *
   * @var \Drupal\Core\Entity\EntityManagerInterface
   */
  protected $entityManager;

  /**
   * Constructs a someClasssForm object.
   *
   * @param \Drupal\Core\Entity\EntityManagerInterface $entity_manager
   *   The entity manager.
   */
  public function __construct(EntityManagerInterface $entityManager) {
    $this->entityManager = $entityManager;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('entity.manager')
    );
  }

  public function someFunction() {
    $build = array();
    $form = $this->entityManager
      ->getFormObject('{ENTITY_TYPE}', 'default')
      ->setEntity($entity);
    $build[] = \Drupal::formBuilder()->getForm($form);

    return $build;
  }
}