Race condition in PollingDirectoryWatcher

[grouma]

I believe there is a race condition here:
https://github.com/dart-lang/watcher/blob/master/lib/src/directory_watcher/polling.dart#L136

Between polling for the list of files to watch and running getModificationTime, the underlying file could have been deleted or moved. This will result in an unhandled failure.

[grouma]

This is likely not an issue.