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

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