private function removeUniqueFile($file)
    {
        if (!isset($this->manifest[$file])) {
            return;
        }

        // delete unique file
        $uniqueFile = public_path($this->manifest[$file]);
        if ($uniqueFile !== null && file_exists($uniqueFile)) {
            unlink($uniqueFile);
        }

        // update manifest
        unset($this->manifest[$file]);
        if (file_put_contents($this->manifestFile, '<?php return ' . var_export($this->manifest, true) . ';' . PHP_EOL, LOCK_EX) === false) {
            throw new RuntimeException(sprintf('Asset Manager was not able to save manifest file "%s"', $this->manifestFile)); // @codeCoverageIgnore
        }
    }