If you are using EF Core 2.1 or later, this capability is available right out of the box.
Asked for in 2010:
When in an data model for entity framework has a navigation property it is not posseble to eager load that navigation property besides when using OfType<> or when eager loading the derived type itself by a navigation property.
Observation from 2018:
The feature is part of EF Core 2.1, which is currently in preview. Please create issues in our issue tracker if you find any problems.