Ensure all of the SignalR client's dependencies are available on netstandard2.0

[natemcmaster]

In manually reviewing the list of packages we are shipping in 3.0 Preview 1, it appears we may be missing some dependencies required by the SignalR client in non-shared-framework scenarios.

Requirements

• Verify that all of Microsoft.AspNetCore.SignalR.Client's dependencies
  • target netstandard2.0
  • ship as packages
• Add tooling to test for this scenario automatically

Notably, it appears we need to fix the packaging for Microsoft.AspNetCore.Connections.Abstractions.

cc @davidfowl

[muratg]

Nate, https://github.com/aspnet/AspNetCore/blob/f0b4ac75c479f207b16331316034910632761e4e/src/Servers/Connections.Abstractions/src/Microsoft.AspNetCore.Connections.Abstractions.csproj#L5

Looks like it's targeting netstandard2.0. Is there an issue with packaging or something?

[natemcmaster]

There appears to be a packaging issue as a result of #4178. The client apparently needs Microsoft.AspNetCore.Connections.Abstractions, but there is not package for this in 3.0 preview 1.

[natemcmaster]

I manually fixed the list, but we should still consider adding better tooling to ensure we don't regress this.

[natemcmaster]

Implementation ideas:

• write a unit test which reads assembly references and checks that nuspec reflects the assembly's runtime dependencies
• write E2E tests for SignalR tests that validate a signalr client without the aspnetcore shared framework

[muratg]

@mikaelm12 feel free to sync-up with me to clarify this.