function exists(path, callback) {
        stat(path, function (err) {
            if (err) {
                if (err === FileSystemError.NOT_FOUND) {
                    callback(null, false);
                } else {
                    callback(err);
                }
                return;
            }

            callback(null, true);
        });
    }
Determine whether a file or directory exists at the given path by calling
back asynchronously with either a FileSystemError string or a boolean,
which is true if the file exists and false otherwise. The error will never
be FileSystemError.NOT_FOUND; in that case, there will be no error and the
boolean parameter will be false.

@param {string} path
@param {function(?string, boolean)} callback