Don't globally disable PHPCS's prohibition of assert()

Don't globally disable PHPCS's prohibition of assert()

Whitelist the remaining usages of assert(), and reinstate the PHPCS sniff
that forbids usage of it. Add FIXME comments as well, so any casual readers
of the code will not think that the disabling and usage is intentional.

Change-Id: I7cabe715c0e6aa6a9ef3ffe5657f3de7fd8e662b