function FileSystemEntry(path, fileSystem) {
        this._setPath(path);
        this._fileSystem = fileSystem;
        this._id = nextId++;
    }
Model for a file system entry. This is the base class for File and Directory,
and is never used directly.

See the File, Directory, and FileSystem classes for more details.

@constructor
@param {string} path The path for this entry.
@param {FileSystem} fileSystem The file system associated with this entry.