Consider adding Nullable annotations to .Net Standard

[wtgodbe]

https://github.com/dotnet/coreclr/tree/NullableFeature

We should consider adding the annotations from the above Feature branch to the .Net Standard for 2.1.

CC @wtgodbe @terrajobst @dotnet/nullablefc

[terrajobst]

.NET Core 3.0 won't have fully annotations for nullable reference types. Instead, we're going to have System.Private.CorLib fully annotated, but right now this a work in progress. Our goal is that we complete the BCL annotations (System.*, probably excluding WinForms/WPF) by .NET 5. As such it's too early to incorporate them into .NET Standard. I'll move this issue to future.

[terrajobst]

Closing as not planned, as .NET Standard is considered complete.