EntityEntry does not contain a definition for 'Reference'

How do I specify in EF7 that I wish to explicitly load the reference property Subject of my ApplicationUser entity? In EF6 the following is defined in the namespace System.Data.EntityFramework.Infrastructure:

public DbReferenceEntry<TEntity, TProperty> Reference<TProperty>(Expression<Func<TEntity, TProperty>> navigationProperty) where TProperty : class;

So this code:

public async override Task<ApplicationUser> FindByIdAsync(string userId, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken))
    {
        var user = await base.FindByIdAsync(userId);
        this.Context.Entry(user).Reference(u => u.Subjects).Load();
        return user;
    }

gives an exception

EntityEntry<ApplicationUser> does not contain a definition for 'Reference'.

Where is this extension method defined in EF7?