Steps to reproduce:
1. Open the attached project
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
6. Build for WSA Universal 10 SDK again
7. Observe there is no error in the Console

Expected result: Unity should show the error
Actual result: Unity does not show the error

Reproduced with: 5.4.5p2, 5.5.3p3, 5.6.0p1, 5.6.0p4, 2017.1.0b3

Fixed in: 2017.1.0b4