To reproduce:
1) Create new project (or use attached project - skip to step 5)
2) Create a new C# script
3) Add an Awake function and this to it: DateTime t = DateTime.Now;
4) Add the script to any object in scene
5) Build and run on WebGL

Expected: It runs normally
Actual: This is thrown - https://i.imgur.com/IMnSsji.png

Reproduced in 2018.1.0b1, 2018.1.0b9
Not reproduced in 2017.3.1p3, 2017.1.0f3

Not reproduced in Windows builds (both IL2CPP and Mono)