To reproduce:
1. Create a new Unity project
2. In Player Settings change Scripting Runtime Version to .NET 4.x Equivalent
3. Open Window > Package Manager and in All tab download Entities package
4. Create a new C# script and open it
5. Add: using UnityEngine.Networking; and change from MonoBehaviour to NetworkBehaviour
6. Observe the console

Expected result: no errors are thrown
Actual result: UNetWeaver error is thrown

Reproduced on:
2018.1.8f1 2018.2.0f2

Not reproduced on:
2018.3.0a4

Notes:
Only reproduced on win 10 machine