org\bovigo\vfs\vfsStreamWrapper::register PHP Method

    public static function register()
    {
        self::$root = null;
        self::$quota = Quota::unlimited();
        if (true === self::$registered) {
            return;
        }
        if (@stream_wrapper_register(vfsStream::SCHEME, __CLASS__) === false) {
            throw new vfsStreamException('A handler has already been registered for the ' . vfsStream::SCHEME . ' protocol.');
        }
        self::$registered = true;
    }

 protected function setFileSystem()
 {
     vfsStreamWrapper::register();
     $root = vfsStream::newDirectory('build');
     vfsStreamWrapper::setRoot($root);
     return $root;
 }