Reason for revert:
Fails a test262 test with --always-opt.

Original issue's description:
> Stage sloppy let
>
> Move the --harmony-sloppy-let flag to staging for further testing, and
> update test262 for the new passing tests. Also increase the strictness
> of the parser, even in sloppy mode, to disallow "new legacy compat" for
>
>   for (let x = 5 in {}) {}
>
> which is now a SyntaxError.
>
> BUG=v8:3305
> LOG=Y
> R=adamk
>
> Committed: https://crrev.com/07bc0117be8dc9e63ec14d5f9645c483d60a1bec
> Cr-Commit-Position: refs/heads/master@{#30515}

TBR=adamk@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:3305

Review URL: https://codereview.chromium.org/1324033002

Cr-Commit-Position: refs/heads/master@{#30518}