This patch puts --harmony-sloppy into staging. Now that let, lexically-scoped
functions and ES2015 sloppy mode const semantics have been split off into
separate flags, the change only enables classes in sloppy mode.

BUG=v8:3305
R=adamk
LOG=Y

Review URL: https://codereview.chromium.org/1288153003

Cr-Commit-Position: refs/heads/master@{#30141}