Well, it's your call of course, but this comes as a surprise for me.
After all, at that point you assume that the file exists.
You [call `file_get_contents`](https://github.com/filp/whoops/blob/5138d11a18e5e0c0aa80a6271ba4d663406a1a2e/src/Whoops/Exception/Frame.php#L92) on that string, which will fail if the file does not exist.
This assert will catch any future problems similar to "Unknown".

But of course, that's just my opinion. I think that there can never be too few asserts. :)
If you insist, I will issue a new pull request without the assert.