DefaultBlockController.php

<?php
/**
 * @file
 * Contains Drupal\block\DefaultBlockController.
 */

namespace Drupal\block;

use Symfony\Component\HttpFoundation\Request;
use Drupal\Core\Page\HtmlFragment;
use Drupal\block\BlockPluginInterface;
use Drupal\Core\Extension\ModuleHandlerInterface;

/**
 * Default controller for rendering a block.
 */
class DefaultBlockController implements BlockControllerInterface {

  /**
   * @var \Drupal\Core\Extension\ModuleHandlerInterface
   */
  protected $moduleHandler;

  public function __construct(ModuleHandlerInterface $moduleHandler) {
    $this->moduleHandler = $moduleHandler;
  }
  /**
   * Renders a provided block into an HtmlFragment.
   *
   * @param Request        $request
   * @param BlockPluginInterface $block
   *
   * @return \Drupal\Core\Page\HtmlFragment
   */
  public function respond(Request $request, BlockPluginInterface $block) {
    $fragment = new HtmlFragment();
    $fragment->addAssetBag($block->getAssets());
    $fragment->setTitle($block->getTitle());

    if ($content = $block->build()) {
      $render = array(
        '#theme_wrappers' => array('block_ng'),
        '#block' => $block,
        'content' => $content,
        '#block_config' => array(
          // @todo most of this doesn't make sense, but it's here to kinda keep things on the rails for now.
          'label' => check_plain($block->getTitle()),
        ),
      );

      $this->moduleHandler->alter('block_ng_view', $render);

      $fragment->setContent(drupal_render($render));
    }

    return $fragment;
  }
}