The plugin supports Swift Version 4.2. Make sure you have this version set in your Build Settings -> SWIFT_VERSION
这个是我从multi_image_picker中找到的说明如何导入，测试成功,我新建了一个flutter swift的，第一种方法。
参考链接：https://github.com/Sh1d0w/multi_image_picker/blob/master/doc/initial-setup.md