Steps to reproduce:
1. Open the attached project (TCPClient.zip)
2. Open the Assets/test.unity scene
3. Switch platform to Windows Store
4. Build for WSA Universal 10 SDK
5. Observe the error in the Console:
The type or namespace name 'TcpClient' could not be found

Expected result: Unity should not show the error
Actual result: Unity shows the errorg

Note:
With Microsoft.NETCore.UniversalWindowsPlatform 5.2.3 everything works as expected

Reproduced with: 5.4.5p2, 5.5.3p3, 5.6.0p1, 5.6.0p4, 2017.1.0b4

Why won't fix: we decided to not spend resources on upgrading .NET Core version used on .NET scripting backend as we focus our efforts on IL2CPP. .NET scripting backend is deprecated starting Unity 2018.2.