Multiple let declaration with es6

[fathyb]

protobuf.js version: 6.7.3

The verify method of messages with multiple messages field gets multiple es6 block-scoped let error declarations.

This causes Uncaught SyntaxError: Identifier 'error' has already been declared on v8.

/**
 * Verifies a Shadow message.
 * @param {Object.<string,*>} message Plain object to verify
 * @returns {?string} `null` if valid, otherwise the reason why it is not
 */
Shadow.verify = function verify(message) {
    if (typeof message !== "object" || message === null)
        return "object expected";
    let error = $root.Color.verify(message.color);
    if (error)
        return "color." + error;
    if (typeof message.blur !== "number")
        return "blur: number expected";
    let error = $root.Point.verify(message.offset);
    if (error)
        return "offset." + error;
    return null;
};

message Shadow {
    required Color color = 1;
    required double blur = 2;
    required Point offset = 3;
}