A file system watcher.

It monitors changes to contents of directories and sends notifications when files have been added, removed, or modified.

As best it can, this package treats valid symlinks as copies of the linked files or directories, and broken symlinks as non-existent. However, there are some caveats. In particular, due to limitations in the native file watching APIs for various platforms, a watcher's behavior is undefined if:

• A target is added for a symlink that was previously broken.

• A symlink links to another symlink, and the second symlink is removed.

• A symlink links to a file or directory contained within another symlink, and the second symlink is removed.

• A symlink's own target changes. This is undefined only when using the polling watcher, due to issue 24821.