function FileSystem() {
        // Create a file index
        this._index = new FileIndex();

        // Initialize the set of watched roots
        this._watchedRoots = {};

        // Initialize the watch/unwatch request queue
        this._watchRequests = [];

        // Initialize the queue of pending external changes
        this._externalChanges = [];
    }
The FileSystem is not usable until init() signals its callback.
@constructor