The project provided uses "Asset Bundle Browser" tool but it has anything to do with the bug

To reproduce:
1) Open the attached project "943438repro"
2) From the menu open Windows -> Asset Bundle Browser
3) Press "Build" to build the Asset Bundle
4) Open "AssetBundleTest.unity" scene
5) Enter play mode
6) On the GUI in the top right corner press "Load Bundle (Async)"
Result: "Assertion failed: Assertion failed on expression: 'Thread::CurrentThreadIsMainThread()" Error is thrown

Note: this can be reproduced without the "Asset Bundle Browser" tool
When using the "Asset Bundle Browser" tool and the script for loading the asset in the project some minor tweaking might be required depending on the platform it is being tested on

Reproducible on both Mac and Windows

Reproduced on: 2017.3.0a5, 2017.2.0b8
Regression introduced with: 2017.2.0b6
Not reproducible on: 2017.2.0b5 and earlier

5.6.3p2