I am not sure if this is the standard practice but it might be of interest to you:

Assuming you have a CS (or related) background and also that your reviewer also would be in a CS related field:

GitHub does provide the ability to have private repositories. You can also add collaborators, to whom you want to grant access to, via their GitHub username or email. (Also, GitHub also provides free private repositories for as long as you are a student. See GitHub Student Pack)

If you are not a student, you can use BitBucket, another version control system that offers free private repositories.

Edit: This gives away the reviewer's identity. (Not Suitable)

I am not sure if this is the standard practice but it might be of interest to you:

Assuming you have a CS (or related) background and also that your reviewer also would be in a CS related field:

GitHub does provide the ability to have private repositories. You can also add collaborators, to whom you want to grant access to, via their GitHub username or email. (Also, GitHub also provides free private repositories for as long as you are a student. See GitHub Student Pack)

If you are not a student, you can use BitBucket, another version control system that offers free private repositories.