Steps to reproduce:
1. Download attached project
2. Switch platform to UWP
3. In the player setting switch backend to IL2CPP and API Compatibility Level to .NET 4.6
4. Build project

Expected results: build is successfully completed
Actual results: build fails with UNetWeaver error

Reproducible just with IL2CPP and API Compatibility Level .NET 4.6

Reproduced with: 2017.4.1f1, 2017.3.2f1, 2017.2.2p3, 2017.2.0b7
Not reproducible with: 2018.2.0b1, 2018.1.0f1, 2017.1.3p3
Regression introduced in: 2017.2.0b7

Example code which triggers this error:
using UnityEngine.Networking;
public class testScript : NetworkBehaviour {
public System.Action testEvent2;
}