Support running tests by absolute file: uri

[jakemac53]

Fixes #1891
Fixes #1899

For any path which looks like it has a scheme (contains "://"), parse it as a uri and grab the path from that. On windows, we also remove any leading / from the path because it will be followed by a drive letter, which we want to be the first part of the path (and not preceded by a /).

Open to better ideas here too, if you have them :)

[jakemac53]

ready for another review @natebosch

[DanTup]

On windows, we also remove any leading / from the path because it will be followed by a drive letter, which we want to be the first part of the path (and not preceded by a /).

You probably know this, but I think if you use .toFilePath() instead of .path you wouldn't need to do this (I've seen using .path on file URIs has been a source of many Windows bugs), although you'll get back the path in Windows form (with backslashes) which is often a good thing but I don't know if it's what you want here.

[DanTup]

@jakemac53 it's not urgent, but so I know when to come back to the Flutter side of this, what's the process for it getting into the Dart SDK? If it's just a case of updating the revision in DEPs I'm happy to send a CL for that, but if there's a process/schedule (or you want to get other changes in with it) I'm happy to wait.

[jakemac53]

It should just be updating the revision in DEPs, I think it might happen automatically now also but I am not sure. Usually we would wait until we publish as well but we probably could publish soon, cc @natebosch

[jakemac53]

We also will want to validate these changes internally first

[DanTup]

Okey, I'll hold off doing anything for a little bit. If you do roll it in (or notice it done automatically) and remember, let me know. Otherwise I'll check back in a little while :-)