EntityFramework Core 1.1.0 missing Include()?

I am using EntityFramework Core 1.1.0. I can query a table and load entities, but the instructions from Microsoft indicates if I want to load relational data, I should use the .Include() function:

https://docs.microsoft.com/en-us/ef/core/querying/related-data

You can use the Include method to specify related data to be included in query results. In the following example, the blogs that are returned in the results will have their Posts property populated with the related posts.

using (var context = new BloggingContext())
{
    var blogs = context.Blogs
        .Include(blog => blog.Posts)
        .ToList();
}

I have no .Include() option.

Any ideas why this is missing or how to load foreign-key relational data?

this.context.Mail
    .Include("Files") // This is missing

I have resorted to explicitly loading relational data. This is fine for small result sets, but as my data sets grow, this is going to cause me grief.

var mails = this.context.Mail.ToList();
mails.ForEach(mail =>
{
    this.context.Entry(mail)              
    .Collection(m => m.Files)
    .Load();
});