Android builds fail when assemblies that utilize iOS code are used in the project, even though the code isn't used directly. This affects projects with Unity IAP enabled in some instances.

Steps to reproduce:
1) Open the attached project (repro.zip).
2) Build for Android.

The build fails; error message appears in the console:
Fatal error in Unity CIL Linker
System.NullReferenceException: Object reference not set to an instance of an object.
at UnityLinker.AddUnresolvedStubsStep.GetTypeModule(TypeReference type, AssemblyDefinition[] assemblies)
at UnityLinker.AddUnresolvedStubsStep.Process()
at Mono.Linker.Steps.BaseStep.Process(LinkContext context)
at Mono.Linker.Pipeline.Process(LinkContext context)
at UnityLinker.UnityDriver.Run()
at UnityLinker.UnityDriver.RunDriver()

and also:
Failed running C:\Program Files\Unity2017.3.0f1\Editor\Data\il2cpp/build/UnityLinker.exe <...>
Exception: C:\Program Files\Unity2017.3.0f1\Editor\Data\il2cpp/build/UnityLinker.exe did not run properly!

This does not reproduce with Mono.
This does reproduce with .NET 4.6 and 3.5 scripting runtime versions. Engine Code Stripping has no effect as well.

Reproduced on:
2017.3.0f1, 2017.2.0f3, 2017.1.0a3, 5.6.4p1, 5.6.2f1

Did not reproduce:
2018.1.0a6, 5.5.5p2, 5.5.0b4

Couldn't test: 5.6.0b4