Sather compiled with Cygwin running on Windows has this behavior:

file ::= FILE::open_for_write("temp.txt") ;
file := file+"\n";
file.close ;

gives you a file of length 2 containing <cr><lf>,  whilst -

file ::= BFILE::open_for_write("temp.txt") ;
file := file+"\n";
file.close ;

gives you a file of length 1.
However, if you really need to use FILE instead of BFILE you can use
"\10" instead
of "\n".  Eg;

file ::= FILE::open_for_write("temp.txt") ;
file := file+"\10";
file.close ;