function createClass() {
	var str = '';
	str += '(function create() {';
	str += 'class CustomError extends Error {';
	str += 'constructor( msg ) {';
	str += 'super( msg );';
	str += '}';
	str += '}';
	str += 'return CustomError;';
	str += '})()';
	return eval( str ); // eslint-disable-line no-eval
}
Creates a CustomError class constructor. Note that we use function generation so that tests may be run in browsers not supporting ES2015 classes. This function may be loaded in non-ES2015 environments, but should only be invoked when ES2015 classes are supported.

@private
@returns {Function} constructor