Ender Dragon Not spawning 15w31a

Put the summary of the bug you're having here

*What I expected to happen was...:The Ender dragon to spawn Normally and be able to fight him just like I would do in 1.8.

*What actually happened was...:The Ender Dragon didn't spawn and the only way I could get him to spawn was go to another world

Steps to Reproduce:
1. It happened naturally and I don't know how to reproduce sorry.
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