Steps to reproduce:
1) Create a new project.
2) Build for iOS with Mono2x API.

Several similar error messages pop up in the console:
Cross compilation job UnityEngine.Networking.dll failed.
UnityEngine.UnityException: Failed AOT cross compiler: /Applications/Unity5.6.0b4/PlaybackEngines/iOSSupport/Tools/OSX/mono-xcompiler-wrapper.sh --aot=full,asmonly,nodebug,static,outfile="UnityEngine.Networking.dll.s" "UnityEngine.Networking.dll" current dir : /Users/vaidas/Downloads/monoTest/Temp/StagingArea/Data/Managed
result file exists: False. Timed out: False

The build fails; build directory is empty. This occurs with all stripping options. Doesn't reproduce with IL2CPP.

Reproduced on:
5.6.0b4, 5.6.0b3

Did not reproduce:
5.6.0b2, 5.5.0p4