includes_url( string $path = '', string $scheme = null )

function includes_url( $path = '', $scheme = null ) {
	$url = site_url( '/' . WPINC . '/', $scheme );

	if ( $path && is_string( $path ) )
		$url .= ltrim($path, '/');

	/**
	 * Filters the URL to the includes directory.
	 *
	 * @since WP-2.8.0
	 *
	 * @param string $url  The complete URL to the includes directory including scheme and path.
	 * @param string $path Path relative to the URL to the wp-includes directory. Blank string
	 *                     if no path is specified.
	 */
	return apply_filters( 'includes_url', $url, $path );
}