Disable classes in sloppy mode unless --harmony-sloppy is set

Also clean up flag names a little.

Baseline: https://codereview.chromium.org/713413003/

R=arv@chromium.org, dslomov@chromium.org
BUG=

Review URL: https://codereview.chromium.org/722203006

Cr-Commit-Position: refs/heads/master@{#25435}