How to reproduce:
1. Create a new project
2. Place sprite to Assets' folder
3. Create Image in the scene (Create -> UI -> Image)
4. Open Animation window
5. Select Canvas
6. Click create on animation window (it will add animator to canvas and create animation)
7. Add new property (Add Property -> Image -> RectTransform -> Rotation)
8. Change the last keyframe to -90 (for some users editor crashes at this step)
9. Drag a sprite from the project view into the Image component (notice that rotation sets back to 0)
10. Try to change the Z rotation of the last keyframe to some other negative value

Actual result: Unity crashes

Reproducible: 5.4.0b21, 5.4.1p1
Not reproducible: 5.4.0b20, 5.5.0b4