isOffset_(b1, b2) {
    return (b1 == 0x17 || b1 == 0x1f) && b2 >= 0x21 && b2 <= 0x23;
  }
Checks if this is a offset control code.
@param {number} b1 Byte 1.
@param {number} b2 Byte 2.
@return {boolean}
@private