Resolve implicit fallthrough in Phabricator

Summary: New implicit fallthrough linter detected a few issues; none of these have behavioral impacts but they can clearly be tightened up. See D1824.

Test Plan: Lint; inspection.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1825