To reproduce:
1. Open attached project "BugRepro.zip"
2. Open C# project with Visual Studio by right clicking on Assets folder and pressing Open C# project
3. In Visual Studio press Build > Build All

Expected result: the project is built
Actual result: getting this error in Visual Studio:
Error CS0234: The type or namespace name 'Xcode' does not exist in the namespace 'UnityEditor.iOS' (are you missing an assembly reference?) (CS0234) (MyAsmDef)

Reproduced on:
2018.1.0a1 2018.3.0a1 2018.3.5f1 2019.1.0b3 2019.2.0a6

Not reproduced on:
2017.4.21f1

Notes:
After building on newer versions, the build will fail on 2017.4 versions.