Thank you, good sir Knight Alex.
I can also confirm this is still not working. I, however, did manage to find a natural 'Mega Spruce Taiga (Hills)' by using AMIDST. However, I had the same issue with the .json file not recording it.
Seeing as no one wants to reopen this report, I created a new ticket last night, MC-42457. Maybe we'll get some attention.
Are you using just one sapling, or four? Sadly, there are no one block DO trees. The only way Dark Oak will grow is if you put down four of them in a 2x2 square, like you were trying to make a large jungle tree. Confused me the first time I tried, too. lol
Edit - Good timing, Tails.