/Applications/Arduino.app/Contents/Java/hardware/arduino/avr/cores/arduino"
Of course changing the AVR UART driver doesn't change the SAMD buffering...
I also am using a SAMD21 board (feather m0) and the HardwareSerial.h file under "/Users/userName/Library/Arduino15/packages/adafruit/hardware/samd/1.2.1/cores/arduino"
That's more like it. However, it seems that the Adafruit SAMD code doesn't HAVE a tx buffer at all.
This is something that was added to the Arduino core about a year ago, and it looks like Adafruit never picked up the edit... (TX buffering for UART using RingBuffer · arduino/ArduinoCore-samd@b24c9e5 · GitHub)
I've submitted UART has no transmit buffer. · Issue #59 · adafruit/ArduinoCore-samd · GitHub
And a similar bug for Sparkfun. Sigh. This is the downside of making it easier to provide 'new' cores rather than derivative cores.
Note that Serial.write() on many SAMD boards goes direct to USB without using the uart driver. (not true of Serial1, of course...)