To reproduce:
1. Open project attached by the user
2. In player settings make sure that Scripting Define Symbols is set to ARK_WEBREQUEST
3. Build it for iOS device

Expected result: Unity successfully builds the project.
Actual result: IL2CPP error for type 'System.Char' in assembly '/Users/user/Downloads/BUGREPORT/Temp/StagingArea/Data/Managed/mscorlib.dll'
Additional information: Build a development build for more information. Index was outside the bounds of the array.

Reproduced with: 5.6.0f1, 5.6.3f1
Wasn't able to test on 5.5.x versions due to 80+ compile errors.

On 2017.1 versions after fixing some compile errors it started throwing other errors:
IL2CPP error for method 'System.IntPtr UnitySteer2D.Behaviors.Fish2D::CreateInstance()' in assembly '/Users/vaidas/Downloads/case_940906-BUGREPORT/Temp/StagingArea/Data/Managed/Assembly-CSharp.dll'
Additional information: Build a development build for more information. Index was outside the bounds of the array.

2017.2 and 2017.3 versions thrown errors with UnityLinker, this issue is already reported.

Backported to: 5.6.3p3, 2017.1.1p2, 2017.2.0b11
Fixed in 2017.3.0b1