Mark some AAST methods as final

Summary: Self-explanatory. I don't think any subclasses should reasonably override these methods.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12292