How to reproduce:
1. Open the attached project that contains Unity IAP 1.20.1
2. In Build Settings, make sure that Target Platform is UWP
3. In Player Settings, make sure that the Scripting Backend is set to .NET
4. Build the project
5. In the Console window, observe that the build was successful, but with Reference rewriter errors
6. Open the build solution file in Visual Studio
7. Build the solution

Expected result: The solution is built successfully. (Also, step 4 does not result in Console errors.)
Actual result: Building the solution fails. (Also, there are Console errors as mentioned in step 5.)

Reproducible with - 2017.4.14f1, 2018.2.14f1, 2018.3.0b8

Notes:
- Not tested on 2019.1, since the issue occurs only when Scripting Backend is .NET (and this Scripting Backend is deprecated in 2019.1).
- The issue does not occur when IAP version is 1.18 or 1.20.0. It was only reproducible on the latest IAP version at the time - 1.20.1.