Steps to reproduce:
1. Download attached project
2. Switch platform to UWP
3. Set scripting backend to .NET
4. Build project

Expected results: Build is successfully completed
Actual results: Build failed. Multiple exceptions are thrown related to System.Net.WebSockets.ClientWebSocket

Reproduced with: 2018.2.0b1, 2018.1.0f1, 2018.1.0a6

Note: not reproducible with IL2CPP

One of the exceptions:
Reference rewriter: Error: type `System.Net.WebSockets.ClientWebSocket` doesn't exist in target framework. It is referenced from StupidFun.Games.Unity3D.dll at StupidFun.WebSocketManager.
UnityEngine.Debug:LogError(Object)
PostProcessWinRT:RunReferenceRewriter() (at C:/buildslave/unity/build/PlatformDependent/MetroPlayer/Extensions/Managed/PostProcessWinRT.cs:1024)
PostProcessWinRT:Process() (at C:/buildslave/unity/build/PlatformDependent/MetroPlayer/Extensions/Managed/PostProcessWinRT.cs:211)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Why won't fix: System.NetWebSockets.ClientWebSocket isn't supported on .NET scripting backend. If you want to use it, switch to IL2CPP scripting backend.