I can't pass a File in FileInfo (overriding getFileStream() for testing)

## 💬 Questions and Help

I want to do widget testing on a CircleAvatar, but to mock the image I need to pass an asset file. But, as I pass File('path/to/mock/img'), it shows me the error:

`Abstract classes can't be instantiated.
Try creating an instance of a concrete subtype.`

And, `file 6.1.2` classifies file as abstract. Then, how can I pass file in getFileStream()?

Is there any work around for this? If so, do suggest.

@renefloor

For questions or help we recommend checking:

- The [Flutter tag in Stack Overflow](https://stackoverflow.com/questions/tagged/flutter)

该 issue 没有指出需要检查的源文件、测试或入口点。首先明确预期结果是代码更改、文档更新还是使用指南；没有提供用于说明何种情况可视为完成的验收标准。

由索引模型根据 Issue 内容生成。