[master] Update dependencies from dotnet/arcade (dotnet/corefx#38474)

[master] Update dependencies from dotnet/arcade (dotnet/corefx#38474)

* Update dependencies from https://github.com/dotnet/arcade build 20190611.2

- Microsoft.DotNet.XUnitExtensions - 2.4.1-beta.19311.2
- Microsoft.DotNet.XUnitConsoleRunner - 2.5.1-beta.19311.2
- Microsoft.DotNet.VersionTools.Tasks - 1.0.0-beta.19311.2
- Microsoft.DotNet.ApiCompat - 1.0.0-beta.19311.2
- Microsoft.DotNet.Arcade.Sdk - 1.0.0-beta.19311.2
- Microsoft.DotNet.Build.Tasks.Configuration - 1.0.0-beta.19311.2
- Microsoft.DotNet.Build.Tasks.Feed - 2.2.0-beta.19311.2
- Microsoft.DotNet.Build.Tasks.Packaging - 1.0.0-beta.19311.2
- Microsoft.DotNet.CodeAnalysis - 1.0.0-beta.19311.2
- Microsoft.DotNet.CoreFxTesting - 1.0.0-beta.19311.2
- Microsoft.DotNet.GenAPI - 1.0.0-beta.19311.2
- Microsoft.DotNet.Helix.Sdk - 2.0.0-beta.19311.2
- Microsoft.DotNet.RemoteExecutor - 1.0.0-beta.19311.2
- Microsoft.DotNet.GenFacades - 1.0.0-beta.19311.2

* Update `where T : object` constraint to be `where T : notnull`

* Update several TODO-NULLABLE comments

Commit migrated from https://github.com/dotnet/corefx/commit/28efaabc4349d11a95f8f31eb932c84f207916ab