1. What happened
In Inspector can't assign prefab from Assets, as a GameObject parameter in script.
The behaviour can be viewed here: http://screencast.com/t/36zhLpgAD

2. How we can reproduce it using the example you attached
- Open scene 1
- Select Main Camera GO
- In its 'Camera Script' script component try assigning some of prefabs from Assets folder.
---> Expected: user is able to select asset. Actual: user isn't able. Only prefabs from scene can be selected.

Note: this is happening on 5.4.0b2 build from green trunk. So it isn't an official build yet.
Regression from 5.4.0b1.