Steps to reproduce:
1) Open the attached project.
2) Build for iOS.

Errors show up in the console:

ArgumentException: The Assembly Triangle is referenced by ProBuilderMeshOps-Unity5 ('Assets/ProCore/ProBuilder/Classes/ProBuilderMeshOps-Unity5.dll'). But the dll is not allowed to be included or could not be found.
UnityEditor.AssemblyHelper.AddReferencedAssembliesRecurse (System.String assemblyPath, System.Collections.Generic.List`1 alreadyFoundAssemblies, System.String[] allAssemblyPaths, System.String[] foldersToSearch, System.Collections.Generic.Dictionary`2 cache, BuildTarget target) (at /Users/Josh/Documents/code/unity/Editor/Mono/AssemblyHelper.cs:143)
UnityEditor.AssemblyHelper.FindAssembliesReferencedBy (System.String[] paths, System.String[] foldersToSearch, BuildTarget target) (at /Users/Josh/Documents/code/unity/Editor/Mono/AssemblyHelper.cs:181)
UnityEditor.HostView:OnGUI()

Exception: /Applications/Unity5.6.0b4/Unity.app/Contents/Tools/UnusedByteCodeStripper2/UnusedBytecodeStripper2.exe did not run properly!

Editor.log shows errors regarding Core.xml:
Unable to find the file for the assembly named 'UnityEngine' specified in link file '/Applications/Unity/PlaybackEngines/iOSSupport/Whitelists/Core.xml'

Reproduced on:
5.6.0b4, 5.5.0p4