New MR specifically for re-adding after code freeze

Closes #3239509

core/misc/polyfills/string.includes.es6.js

+/**
+ * @file
+ * Provides a polyfill for String.includes().
+ *
+ * This is needed for Internet Explorer 11 and Opera Mini.
+ *
+ * This has been copied from MDN Web Docs code samples. Code samples in the MDN
+ * Web Docs are licensed under CC0.
+ *
+ * @see https://web.archive.org/web/20210916035058/https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/includes
+ * @see https://developer.mozilla.org/en-US/docs/MDN/About#Code_samples_and_snippets
+ */
+/* eslint-disable strict, lines-around-directive, no-extend-native */
+if (!String.prototype.includes) {
+  String.prototype.includes = function (search, start) {
+    'use strict';
+
+    if (search instanceof RegExp) {
+      throw TypeError('first argument must not be a RegExp');
+    }
+    if (start === undefined) {
+      start = 0;
+    }
+    return this.indexOf(search, start) !== -1;
+  };
+}