public function setFiles($files, $checkLastModified = true)
    {
        $this->_groupKey = null;
        if ($checkLastModified) {
            $this->_lastModified = self::getLastModified($files);
        }
        // normalize paths like in /min/f=<paths>
        foreach ($files as $k => $file) {
            if (0 === strpos($file, '//')) {
                $file = substr($file, 2);
            } elseif (0 === strpos($file, '/') || 1 === strpos($file, ':\\')) {
                $file = substr($file, strlen(self::app()->env->getDocRoot()) + 1);
            }
            $file = strtr($file, '\\', '/');
            $files[$k] = $file;
        }
        $this->_filePaths = $files;
    }