Steps to reproduce:
1. Open attached project.
2. Open script named "Test".
3. Notice that Start() method is trying to return data of class inside DLL.
3. Open scene named "Cena".
4. Press play button and notice output of the "Console" window: "test2 : {}"
(empty curly brackets indicate that JsonUtility.ToJson() did not return object's data).

Expected result:
JsonUtility.ToJson() should return data of class inside DLL.

Reproduced with:
5.3.4f1, 5.3.4p4, 5.4.0b16.