Stack Overflow is part of the Stack Exchange Network, and article 3 of the Terms of Service tell us that “You agree that all Subscriber Content that You contribute to the Network is perpetually and irrevocably licensed to Stack Exchange under the Creative Commons Attribution - Share Alike license.” (http://stackexchange.com/legalhttps://stackexchange.com/legal)
Maybe I overlooked, but I don't see any changes to the Terms of ServiceTerms of Service that mention the MIT License for source code. The most recent question on "Open Source beta" still talks about the CC-BY-SA only: Must CC BY-SA be included for ideas and code taken from Stack Overflow?