EF 7 (Beta 7) One-to-One Relationship

I've noticed questions that have answered this problem for earlier beta versions of EF 7 (like here), but I haven't seen it solved for Beta 7, so here it goes:

I have 2 entities, simplified as follows:

public class FirstEntity
{
    public int FirstEntityID { get; set; }
    /*
        Other fields here
    */
    public int? SecondEntityID { get; set; }
    public SecondEntity SecondEntityProperty { get; set; }
}

public class SecondEntity
{
    public int FirstEntityID { get; set; } 
    /*
        Other fields here
    */
    public FirstEntity FirstEntityProperty { get; set; }
}

The way to map everything has changed so much from earlier versions. How do I map these two entities in a One-to-One relationship?