function initWatchers(changeCallback, offlineCallback) {
        _changeCallback = changeCallback;
        _offlineCallback = offlineCallback;

        if (_isRunningOnWindowsXP && _offlineCallback) {
            _offlineCallback();
        }
    }
Initialize file watching for this filesystem, using the supplied
changeCallback to provide change notifications. The first parameter of
changeCallback specifies the changed path (either a file or a directory);
if this parameter is null, it indicates that the implementation cannot
specify a particular changed path, and so the callers should consider all
paths to have changed and to update their state accordingly. The second
parameter to changeCallback is an optional FileSystemStats object that
may be provided in case the changed path already exists and stats are
readily available. The offlineCallback will be called in case watchers
are no longer expected to function properly. All watched paths are
cleared when the offlineCallback is called.

@param {function(?string, FileSystemStats=)} changeCallback
@param {function()=} offlineCallback