Printing to /dev/stderr doesn't work

Minimal test case:

``` c
#include <stdio.h>

int main() {
    FILE *err = fopen("/dev/stderr", "w");
    fprintf(err, "Hello, stderr!\n");
    return 0;
}
```

When I run this code in nodejs or spidermonkey, nothing is printed to stderr.
However, printing directly to the builtin `stderr` `FILE*` works.
