Dart 2.5.0 seems to break filesystem watching on Windows

[nex3]

Dart Sass's filesystem watcher tests have started failing more consistently on Windows only with the release of Dart 2.5.0. Watcher tests have always been kind of flaky, but we're seeing much more consistent failures now. See this Travis job and this one as examples; both of these jobs were manually restarted about four times, with similar timeouts each time. Note that the latter also increases the timeout and adds test-package-level retries, but the tests are still failing.

However, the most recent master branch build with Dart 2.4 produced no Windows failures, nor did the same pull request with the Windows builder pinned to Dart 2.4.1. This strongly suggests that something changed in Dart 2.5.0 that dramatically increased the flakiness of filesystem watchers on Windows.

This is high-severity for Dart Sass; until this is fixed, we'll need to pin our Windows releases to 2.4.1 so we don't break filesystem watching for downstream users.

[nex3]

As another data point, these same builds were not failing on Node.js on Windows, which ultimately bottoms out on the same OS-level APIs that Dart uses. This further suggests something's up in dart:io.

[zichangg]

@nex3 Hi, looks like watcher doesn't response and trigger a timeout. It is similar to the other issue that I worked on watcher issue and I landed the change about a week ago(https://dart-review.googlesource.com/c/sdk/+/119524).
Can you try it again to see whether my cl solves the problem? If not, I can take a look again.

[nex3]

Is that change in a released dev version of Dart 2.5?

[zichangg]

I'm not quite sure.
Looks like this is added into 2.6.0-dev.7.0, according to 2.6.0-dev.7.0...master.

[nex3]

I'm unable to test this on Dart 2.6.0-dev.7.0 because it appears that it has no x64 Windows release available.

[nex3]

Okay, I managed to figure it out, and it looks like the issue is fixed. 👍

[zichangg]

Thanks for testing. Appreciate! ^_^