WIP: Add a MemoryFileSystemIOOverrides implementation

[jamesderlin]

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Add an IOOverrides implementation for MemoryFileSystem. This provides a less invasive mechanism for testing code that uses dart:io directly.

This is a work-in-progress because:

• The tests probably could be more thorough.
• The tests depend on the fix to FileSystem.isLink (https://github.com/google/file.dart/pull/214).
• The Directory.current setter is broken when using IOOverrides (https://github.com/dart-lang/sdk/issues/52140). I would not be comfortable suggesting using IOOverrides without a fix.
• The above bug also leads me to believe that IOOverrides might not be well-tested. Is using IOOverrides an approach that package:file should endorse, or would it better in some less-official package?

Before I do more work on this, does anyone have any opinions or feedback?

[jakemac53]

@jamesderlin did you want to push on this for the 7.x release? Or should we release that now and do this in a follow up?

[jamesderlin]

No, this draft PR is meant just to solicit feedback about whether this is desirable (and if so, whether it should be in package:file or in its own package).

It's not meant to be breaking, so it definitely can wait. I also wouldn't consider it usable until https://github.com/dart-lang/sdk/issues/52140 is fixed.

[bsutton]

definitely something that I need to help with DCli testing.

[devoncarew]

Thanks for the PR! The source-of-truth for this package has moved to https://github.com/dart-lang/tools; we're closing the PRs in this repo. If you believe that this PR is still relevant, feel free to re-create it against the new location. Thanks!