Steps to reproduce:

1. Create new project
2. Create new scene and save it as "repro"
3. Add empty game object
4. Select it and open animation window
5. Create new animation
6. Save scene "repro" using ctrl+s
7. Reopen scene "repro"
8. Select game object that was created in step 3
9. Notice that it has no Animator component attached

Workaround: Attach animator manually after scene is reopened.

Not reproducible: 5.2.4f1
Reproduced with: 5.3.2p4, 5.4.0b7