When I try to open a world not created in 1.8, it is stuck on the "Downloading terrain" screen.

Put the summary of the bug you're having here

What I expected to happen was...:
For my world to load

What actually happened was...:
Minecraft was stuck on the Downloading terrain screen

Steps to Reproduce:
1. Launch minecraft
2. open singleplayer world made before 1.8
3. ...