undefined variable $faker.

这是WithFaker文件.

https://github.com/laravel/framework/blob/5.5/src/Illuminate/Foundation/Testing/WithFaker.php

<?PHP

namespace Tests\Unit;

use App\User;
use Tests\TestCase;
use Illuminate\Foundation\Testing\WithFaker;
use Illuminate\Foundation\Testing\RefreshDatabase;

class LoginTest extends TestCase
{

    use WithFaker;

    /**
     * A basic test example.
     *
     * @return void
     */

    /** @test */
    public function test_example()
    {

        $user = User::create([
            'username' => $faker->firstName(),]);

    }

}