Steps to reproduce:
1) Download attached project "AssemblyBug.zip" and open in Unity
2) Make sure platform is set to Android
3) Ctr+Shift+B >> Player Settings >> Other Settings >> Configuration >> Scripting Backend >> make sure its IL2CPP
4) Build and run scene "123"

Note: Notice that Unity fails to build this project and throws ' AssemblyResolutionException' error in console (see attached image "Error.png")
Note: Works fine with Mono2x

Also reproduces with WebGL

Reproduced with:
5.2.4f1, 5.3.6p8, 5.4.2p4, 5.5.0b11, 5.6.0a1