From Capcom Blogs:
For a lot of us, the Legend of Sheng Long is like the Contra code–part of the very fabric of our gaming history. It's not as well-known in Japan, however, so project manager Natsuki Shiozawa breaks it down on Capcom Japan's SFIV blog:
"So who's Sheng Long?" Let me explain.
It all goes back to when Street Fighter II was released in 1991. You all remember Ryu's line, right? "You must defeat my dragon punch to stand a chance!" (It seems like a lot of you do.)