public function add($class, $file)
    {
        $class = $this->normalizeClassName($class);

        if ($this->has($class)) {
            if (($localFile = $this->getFileFor($class)) !== $file) {
                throw new ClassAlreadyRegisteredException($class, $localFile);
            }
        }

        $this->classes[$class] = $file;

        return $this;
    }