Fix race condition in PollingDirectoryWatcher

[grouma]

I'll merge https://github.com/dart-lang/watcher/pull/70/files once it lands.

This closes #69

[natebosch]

Do we want to change how this is handled at

watcher/lib/src/file_watcher/polling.dart

Line 65 in 562b9e5

modified = await getModificationTime(path);

[grouma]

I don't think we want to change the logic in the file_watcher. A user is explicitly providing a path to watch and therefore would like want to be notified of watch errors. The directory watcher is using implicit file paths due to the polling logic.

[natebosch]

A user is explicitly providing a path to watch and therefore would like want to be notified of watch errors

I don't think it would be considered an error. We handle the removed case at

watcher/lib/src/file_watcher/polling.dart

Lines 57 to 61 in 562b9e5

	if (_lastModified != null && !pathExists) {
	_eventsController.add(WatchEvent(ChangeType.REMOVE, path));
	unawaited(close());
	return;
	}

- we just have a potential race condition for if the file is removed in between our check for exists and our file stat.