io: fix newline on custom output port

Closes #2677

pkgs/racket-test-core/tests/racket/file.rktl

@@ -1218,6 +1218,17 @@
     (test (void) close-output-port p)
     (test 10 values l)))
 
+(let ()
+  (define-struct myport (port)
+    #:property prop:output-port 0)
+  (define o (open-output-string))
+
+  (define out (make-myport o))
+  (test (void) newline out)
+  (test "\n" get-output-string o)
+
+  (err/rt-test (newline 9)))
+
 ; --------------------------------------------------
 
 (load "tmp1")

racket/src/io/print/main.rkt

@@ -127,8 +127,7 @@
           (void))))
 
 (define/who (newline [o (current-output-port)])
-  (check who output-port? o)
-  (unsafe-write-bytes 'newline #"\n" o)
+  (unsafe-write-bytes 'newline #"\n" (->core-output-port o who))
   (void))
 
 ;; ----------------------------------------