Issue #2196807 by longwave, divesh.kumar: Convert all calls to check_plain()...

Issue #2196807 by longwave, divesh.kumar: Convert all calls to check_plain() in core to Drupal\Component\Utility\String::checkPlain() in block module.

core/modules/block/lib/Drupal/block/BlockViewBuilder.php

@@ -7,6 +7,7 @@
 
 namespace Drupal\block;
 
+use Drupal\Component\Utility\String;
 use Drupal\Core\Entity\EntityViewBuilder;
 use Drupal\Core\Entity\EntityViewBuilderInterface;
 use Drupal\Core\Entity\EntityInterface;
@@ -61,7 +62,7 @@ public function viewMultiple(array $entities = array(), $view_mode = 'full', $la
           '#base_plugin_id' => $base_id,
           '#derivative_plugin_id' => $derivative_id,
         );
-        $build[$key]['#configuration']['label'] = check_plain($configuration['label']);
+        $build[$key]['#configuration']['label'] = String::checkPlain($configuration['label']);
 
         // Place the $content returned by the block plugin into a 'content'
         // child element, as a way to allow the plugin to have complete control