[RFC] [cross_file] New architecture.

[ditman]

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This PR attempts to solve the current issues of XFile by slightly re-architecting it.

In this PR:

• XFile is now an interface that does not have any code, except for the two "legacy" constructors that are maintained (but deprecated) for backwards-compatibility reasons.
• Each underlying data structure contains a super simple implementation of the interface.
• XFile instances are now created from the platform-specific (native vs web) factories.

This does not remove saveAs from the XFile interface.

[!NOTE] ~~This is a breaking change because I've made the old XFile constructors deprecated (and throwy) but if we want to, we could possibly re-implement the "old" constructors with the new implementations and a few conditional imports?~~

This is not a breaking change anymore. Old tests continue passing (test/legacy*), and new tests contain minor changes.

Anyway, just a RFC!

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene page, which explains my responsibilities.
• [x] I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• [x] I signed the CLA.
• [x] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [ ] I linked to at least one issue that this PR fixes in the description above.
• [x] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• [x] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I added new tests to check the change I am making, or this PR is test-exempt.
• [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[ditman]

/cc @stuartmorgan this is my idea for the "next" version of the x-files. PTAL and let me know what you think, or if you want this to go in another direction :)

[stuartmorgan-g]

Quick note to say I haven't forgotten about this, I just haven't had time to fully context switch to give this the attention it will need. I'll try to do it within the next week.

[ditman]

I'm going to work on this on my next flutter/packages on-call rotation.

[Piinks]

Hey @ditman! Greetings from stale PR triage. :) Do you plan to pick this back up?

[ditman]

@Piinks I do want to pick this up again, but I don't think I'll have time to do this before I/O :(

[Coinners]

sorry @ditman to bother you but do you plan on finishing this PR

[Piinks]

Hey @ditman! Greetings from stale PR triage (again). Should we continue to leave this open?

[Piinks]

Given the many return trips to stale PR triage, the many merge conflicts, and the lack of progress for a year now, I am going to close this PR. Please feel free to reopen if needed @ditman. Thank you!