function syncUnopenWorkingSet() {
        // We only care about working set entries that have never been open (have no Document).
        var unopenWorkingSetFiles = MainViewManager.getWorkingSet(MainViewManager.ALL_PANES).filter(function (wsFile) {
            return !DocumentManager.getOpenDocumentForPath(wsFile.fullPath);
        });

        function checkWorkingSetFile(file) {
            var result = new $.Deferred();

            file.stat(function (err, stat) {
                if (!err) {
                    // File still exists
                    result.resolve();
                } else {
                    // File has been deleted externally
                    if (err === FileSystemError.NOT_FOUND) {
                        DocumentManager.notifyFileDeleted(file);
                        result.resolve();
                    } else {
                        // Some other error fetching metadata: treat as a real error
                        console.log("Error checking for deletion of " + file.fullPath, err);
                        result.reject();
                    }
                }
            });
            return result.promise();
        }

        // Check all these files in parallel
        return Async.doInParallel(unopenWorkingSetFiles, checkWorkingSetFile, false);
    }
Scans all the files in the working set that do not have Documents (and thus were not scanned
by findExternalChanges()). If any were deleted on disk, removes them from the working set.