Mark some ActionScript methods as `final`.

Summary: These methods shouldn't be overridden. Marking them as `final` will enforce this.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9411