'jag monitor' doesn't always flush output on Windows

[kasperl]

Reported by @muhlpachr. It seems 'jag monitor' hangs sometimes on Windows:

PS C:\Users\michal\test> jag monitor -a
Starting serial monitor of port 'COM11' ...
o world!
158 Hello world!
159 Hello world!
160 Hello world!
1

Serial port seams to be fine, when I connect with terminal emulator or from Python, it continues without problem and do not hang.

Just for reference used toit code is this simple:

main:
  i := 1
  while true:
    print "$i Hello world!"
    i++
    sleep --ms=500

Originally posted by @muhlpachr in https://github.com/toitlang/toit/issues/457#issuecomment-1040926507