Add assertion that checkpoints have snapshots
Add assertion that checkpoints have snapshots

Signed-off-by: Nathan Sobo <2e8aa918660411855c6d44d5bb2da677aa033255@github.com>

pombredanne/text-buffer,pombredanne/text-buffer,jacekkopecky/text-buffer,atom/text-buffer,atom/text-buffer,whodatninja/text-buffer,jacekkopecky/text-buffer