Minecraft 15w31b, Minecraft 15w31c, Minecraft 15w32a
Windows 10 Pro 64-Bit, Java 1.8.0_25
Minecraft 15w31b, Creative Mode
UPDATE: This has been fixed in 15w32b. Video: https://youtu.be/2CcZTfdY0js
When fighting the dragon, it cannot die unless it is perched on the End Portal. Only tested with enchanted diamond sword (sharpness 5) in creative mode. Untested with bow or in survival mode.
More details: If you perform the final hit on the dragon while it is flying around, it will make the regular hitting noise instead of the dragon's hit noise. Once you hit it a singular beam of light comes out of its chest, staying there until it dies properly. If you hit it while perched, it will go through the regular death animation (may I also mention that the portal spawns above the existing one, but that's a separate issue).
I recorded a video of this here: http://youtu.be/elT-k0gh56o