one of the io doc tests fails if port 8080 is already bound to

[nickdesaulniers]

@alexcrichton helped me find this the hard way. was running vlc by accident. this test

The test is a should fail, but doesn't since another process is listening on the port.

[alexcrichton]

The test should probably just fail!() immediately. We just want to compile it make sure the syntax/types are valid.