In the method net.minecraft.entity.boss.dragon.EnderDragonEntity.kill() it sets the entity dead and processes the death (portal respawn, ect) while in the method net.minecraft.entity.boss.dragon.EnderDragonEntity.damagePart(), if killed by damage source; it sets the health to 1 (so it can fly to the bedrock portal) and phase to DYING. Code-wise, it seems intended but if not, the code in the method kill() should be changed to same as the part in the method damagePart() if the dragon' health is 0.
Confirmed in 1.8.8 and 15w31a.