Laravel exception 'ReflectionException' with message 'Class validator does not exist' in

Laravel 不能使用Validator。

# app/Providers/AppServiceProvider.php
    public function boot()
    {

        Validator::extend('cellphone', function($attribute, $value, $parameters, $validator) {
            return preg_match('/^1\d{10}$/', $value);
        });

    }

报错

exception 'ReflectionException' with message 'Class validator does not exist' in