isBackgroundAttribute_(b1, b2) {
    // For Background Attribute Codes, the bytes take the following form:
    // Bg provided: b1 -> |0|0|0|1|C|0|0|0| b2 -> |0|0|1|0|COLOR|T|
    // No Bg:       b1 -> |0|0|0|1|C|1|1|1| b2 -> |0|0|1|0|1|1|0|1|
    return (((b1 & 0xf7) === 0x10) && ((b2 & 0xf0) === 0x20)) ||
             (((b1 & 0xf7) === 0x17) && ((b2 & 0xff) === 0x2D));
  }
Checks if this is a background attribute control code.
@param {number} b1 Byte 1.
@param {number} b2 Byte 2.
@return {boolean}
@private