I created a new world with the latest snapshot, and started terraforming a world, when I noticed the dirt wouldnt fill with grass. I had the tickspeed set to 1000, I tried various other tickspeeds, but I had the same issue.
What I expected to happen was...:
The dirt blocks adjacent to grass blocks should be turning into grass blocks.
What actually happened was...:
The dirt blocks adjacent to grass blocks stayed as dirt blocks, no matter my tickspeed.
Steps to Reproduce:
1. Create a new world
2. Find a hill of grass
3. Destroy the grass blocks until some dirt is visible, and turn the tickspeed up to 1000 or some other large number.
Grass only spreads when there is light; nighttime does not provide enough light.