Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

vendor/masterminds/html5/test/HTML5/Parser/InstructionProcessorMock.php

@@ -0,0 +1,26 @@
+<?php
+namespace Masterminds\HTML5\Tests\Parser;
+
+class InstructionProcessorMock implements \Masterminds\HTML5\InstructionProcessor
+{
+
+    public $name = null;
+
+    public $data = null;
+
+    public $count = 0;
+
+    public function process(\DOMElement $element, $name, $data)
+    {
+        $this->name = $name;
+        $this->data = $data;
+        $this->count ++;
+
+        $div = $element->ownerDocument->createElement("div");
+        $div->nodeValue = 'foo';
+
+        $element->appendChild($div);
+
+        return $div;
+    }
+}