node_modules/core-js/modules/_string-context.js

// helper for String#{startsWith, endsWith, includes}
var isRegExp = require('./_is-regexp');
var defined = require('./_defined');

module.exports = function (that, searchString, NAME) {
  if (isRegExp(searchString)) throw TypeError('String#' + NAME + " doesn't accept regex!");
  return String(defined(that));
};