Issue #915756: Add masquerade in admin menu and create a page

src/Controller/MasqueradeBlockController.php

+<?php
+
+namespace Drupal\masquerade\Controller;
+
+use Drupal\Component\Plugin\PluginManagerInterface;
+use Drupal\Core\Controller\ControllerBase;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Returns responses for Masquerade routes.
+ */
+class MasqueradeBlockController extends ControllerBase {
+
+  /**
+   * The plugin.manager.block service.
+   *
+   * @var \Drupal\Component\Plugin\PluginManagerInterface
+   */
+  protected $pluginManagerBlock;
+
+  /**
+   * The controller constructor.
+   *
+   * @param \Drupal\Component\Plugin\PluginManagerInterface $plugin_manager_block
+   *   The plugin.manager.block service.
+   */
+  public function __construct(PluginManagerInterface $plugin_manager_block) {
+    $this->pluginManagerBlock = $plugin_manager_block;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('plugin.manager.block')
+    );
+  }
+
+  /**
+   * Returns the masquerade block.
+   */
+  public function getMasqueradeBlock() {
+
+    // Create a masquerade block instance.
+    $build['masquerade'] = $this->pluginManagerBlock
+      ->createInstance('masquerade')
+      ->build();
+
+    return $build;
+  }
+
+}