Make ControllerBase.User virtual for unit testability.

[mkArtakMSFT]

This issue is to track a repo wide impacting changes:

Remove the compatibility switches

• Remove compat switch for Enum DataAnnotation localization. In Update package branding to 3.0.0-preview4 #7748 we decided to fix the fact that Enum localization didn't use DataAnnotationLocalizerProvider behind a compat switch because it was a breaking change. For 3.0.0 we should remove that compat switch and only support the new behavior (usage of DataAnnotationLocalizerProvider).

Remove all the obsolete methods

• See Remove obsolete API aspnet/Mvc#6497

Unit testing cleanup

• Make ControllerBase.User virtual for unit testability.

[dougbu]

This looks like the uber-issue for 3.0 😈 But, do we have an issue covering removal of [Obsolete] types and members?

I see #7382 but nothing that covers all of the ~38 [Obsolete] members e.g. the many old constructor overloads in input formatters and model binders or the ActionResult.ExecuteResult(...) overrides.

[mkArtakMSFT]

@pranavkm, what is left here? Are some of these listed items done already?

[pranavkm]

Make ControllerBase.User virtual for unit testability.

I think that's the only one. I'll update the title