Steps to reproduce:
1. Open attached project
2. Open "AssemblyLoadBug" scene
3. Enter play mode and observe console
*assembly is loaded and its type is successfully retrieved*
4. Exit play mode
*at this moment the main assembly gets unloaded(without forcibly unloading it)*
5. Enter play mode and observe console

Actual result: the main assembly is unloaded and getting its type causes a failure (Editor needs to be restarted)

Notes:
Only reproducible when using Assembly.Load from bytes (alternatively restart Editor, select "AssemblyLoadBug" in the hierarchy and change "Method" value to 'File')
Not reproducible with .NET 3.5 scripting runtime version

Reproducible with: 5.5.0b9, 2017.1.0b1, 2017.1f1, 2017.2.0b1

Fixed in: 2017.1.0p4, 2017.2.0b6, 2017.3.0a3, 2018.1.0b6