Steps to reproduce:

1. Open attached project
2. In console, notice errors:
- Assets/Test.cs(8,9): error CS0246: The type or namespace name 'WWW' could not be found (are you missing a using directive or an assembly reference?)
- Assets/Test.cs(9,9): error CS0246: The type or namespace name 'ParticleSystem' could not be found (are you missing a using directive or an assembly reference?)

With .NET 3.5 it instead shows this as expected:
- Assets/Test.cs(8,9): error CS1070: The type `UnityEngine.WWW' has been forwarded to an assembly that is not referenced. Enable the built in package 'Asset Bundle' in the Package Manager window to fix this error.

Reproduced with: 2018.3.0b1, 2018.3.0b3
Not reproducible: 2018.2.10f1