How to reproduce:
1. Create a new project or open the attached one
2. Make sure that scripting Backend is set to Mono
3. Build iOS build
Result: buid fails with an error: "Cross compilation job UnityEngine.Advertisements.dll failed.
UnityEngine.UnityException: Failed AOT cross compiler: /Applications/Unity 2018.2.0a8/PlaybackEngines/iOSSupport/Tools/OSX/mono-xcompiler-wrapper.sh --aot=full,asmonly,nodebug,static,outfile="UnityEngine.Advertisements.dll.s" "UnityEngine.Advertisements.dll" current dir : /Users/ieva/Downloads/EmptyTestMono/Temp/StagingArea/Data/Managed
result file exists: True. Timed out: False"

Note: Removing Ads Packages solves the issue

Reproducible: 2018.2.0a6, 2018.2.0a8

Not reproducible: 2018.2.0a5 <- build was failing because of different reason "Shader error in 'Hidden/PostProcessing/SubpixelMorphologicalAntialiasing': 'mad' : no matching overloaded function found at /Users/ieva/Library/Unity/cache/packages/packages.unity.com/com.unity.postprocessing@0.1.7/PostProcessing/Shaders/Builtins/SubpixelMorphologicalAntialiasing.hlsl(1117) (on gles)"

This issue is no longer reproducible: 2018.2.0b2