Steps to Reproduce:
1. Create any 3D game object.
2. In the Inspector, Mark it as static.
3. Drag it to Project window to create Prefab.

NOTE : Observe "Assertion failed: Assertion failed on expression: 'kValidSceneObjectIdentifier == res'" error spamming in Console.

Notes:
- Appears on both Windows and OSX
- Appears on newly created Prefabs as well.

Reproduced on Unity 2017.3.0b1, 2017.3.0b8 and 2018.1.0a3
Regression on Unity 2017.3.0b1
Not reproduced on Unity 5.6.4f1, 2017.2.0f3

NOTE: moving previous steps reported by Milian to the comments section as mine are more general and don't require any additional project.