Dragging a script on a prefab instance prompts a "Losing Prefab" dialog

What happened
Draging a script on a prefab instance prompts a "Losing Prefab" dialog

How we can reproduce it using the example you attached
1. Load the project
2. Create an instance of "Cube" prefab in the scene
3. Try to add the "script" script to the instance by Drag n Drop script on the instance or directly in the inspector having the instance selected.
Result: a "Losing Prefab" dialog is prompted

Notes:
- This is a regression in 2017.1
- Clicking "Cancel" on the dialog still adds the script component anyway
- Create the script from the "Add Component" button doesn't prompt the dialog