unexpected(pos) {
            let message = "Unexpected token";

            if (pos !== null && pos !== void 0) {
                this.pos = pos;

                if (this.options.locations) {
                    while (this.pos < this.lineStart) {
                        this.lineStart = this.input.lastIndexOf("\n", this.lineStart - 2) + 1;
                        --this.curLine;
                    }
                }

                this.nextToken();
            }

            if (this.end > this.start) {
                message += ` ${this.input.slice(this.start, this.end)}`;
            }

            this.raise(this.start, message);
        }
Overwrites the default unexpected method to throw Esprima-style errors.
@param {int} pos The position of the error.
@throws {SyntaxError} A syntax error.
@returns {void}