phpunit – 在单元测试中模拟时未找到Laravel外观类

// Helper class

<?php namespace MyAppHelpers;

    use User;
    use File;

    class FileSystemHelper
    {
        protected $userBin = 'users/uploads';

        public function createUserUploadBin(User $user)
        {
            $path = $this->userBin . '/' . $user->id;

            if ( ! File::isDirectory($path))
            {
                File::makeDirectory($path);
            }
        }
    }

以及相关的测试：

// Associated test class

<?php 

    use MyAppHelpersFileSystemHelper;

    class FileSystemHelperTest extends TestCase {

        protected $fileSystemHelper;

        public function setUp()
        {
            $this->fileSystemHelper = new FileSystemHelper;
        }

        public function testNewUploadBinCreatedWhenNotExists()
        {
            $user = new User; // this would be mocked

            File::shouldReceive('makeDirectory')->once();

            $this->fileSystemHelper->createUserUploadBin($user);
        }
    }

但是在运行测试时出现致命错误：

PHP Fatal error: Class ‘File’ not found in /my/app/folder/app/tests/lib/myapp/helpers/FileSystemHelperTest.php

我看过嘲笑门面的文档,我看不出我哪里出错了.有什么建议？

谢谢