Use Teensy to program ESP8266

SerialReflector.ino

/**
   SerialReflector
*/

void setup() {
  Serial.begin(115200); // Serial port for connection to host
  Serial1.begin(115200); // Serial port for connection to serial device
}
void loop() {
  if (Serial1.available())   {
    Serial.write(Serial1.read());
  }
  if (Serial.available())   {
    Serial1.write(Serial.read());
  }
}