Fix PollingFileWatcher.ready for files that don't exist

[DanTup]

There were a few issues here:

• FileWatcher.ready never fired for files that don't exist because of logic inside FileWatcher existing early if the modification time was null
• The test I recently added trying to catch this was incorrectly passing because the mock timestamp code was set so that files that had not been created would return a 0-mtime whereas in the real implementation they return null

So this change

a) updates the mock to return null for uncreated files (to match the real implementation) which breaks a bunch of tests

b) fixes those tests by updating FileWatcher._poll() to handle null mtime separately from being the first poll.

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[DanTup]

There's a failure here:

❌ test/directory_watcher/polling_test.dart: subdirectories notifies when a subdirectory is moved within the watched directory and then its contents are modified (failed)
  Expected: should emit an event that is modify new/file.txt
    Actual: <Instance of 'StreamQueue<WatchEvent>'>
     Which: emitted • add /tmp/dart_test_NLELVH/new/file.txt
  
  test/directory_watcher/shared.dart 250:7  sharedTests.<fn>.<fn>

This happens because now the timestamp code is "fixed", this rename looks like a new folder because renameDir() doesn't migrate the mock timestamps. I changed that but it broke many other tests so it need a little more work.

[DanTup]

Pushed some additional changes:

• renameDir moves the timestamps over for files it knows about
• mock timestamps are now an increasing number starting from 1
  This is because otherwise, tests that delete and re-create a file could result in the same timestamp for the pre-delete file and the new file, and depending on timing that could look like the file never changed (whereas now it'll either by a delete+add or a modify, because of the timestamp change).
  This wasn't an issue before because deleted files didn't have their timestamps cleared and the re-add would still increase them (but now we delete timestamps for deleted files)

[DanTup]

@jakemac53 this ended up touching a little more than originally planned, but I think it all makes sense and should result in the tests running with behaviour that much better matches the real fs (and ofc, doesn't mask issues like dart-lang/sdk#54116) :-)

Please review carefully - I'd like to roll this into the SDK once merged :-)

[jakemac53]

I do think we should wait to land/publish this until after the thanksgiving break if that is alright

[DanTup]

@jakemac53 yeah, that's fine by me. It isn't urgent - I'd just like to ensure I do some final testing once it gets into the analyzer and not forget about it after it lands :)

[DanTup]

@jakemac53 how about now? :) still not urgent, just still on my list to not forget about 😄

[jakemac53]

I will circle back soon, lots of things to catch up on 🤣

[jakemac53]

@DanTup it sounds like you are going to try merging this into the SDK prior to publishing?

[DanTup]

@jakemac53 I've opened https://dart-review.googlesource.com/c/sdk/+/339401 to roll this into the SDK, and have confirmed locally the failing test now passes.

I don't need the Pub package, so I'll leave it to you to decide when to publish. Thanks! :-)