See the discussion at #6055 (comment)

Basically, we don't really want to have interfaces for this sort of thing. But currently we do because:

• When @dougbu tried to not have an interface for this before, he encountered some issue to do with overridable setters or something - not sure of the details
• Removing the interface would mean moving the .resx file from the Mvc.Core project to the Abstractions project, and figuring out whether this has effects on build or localisation