Stop emitting kSloppyLexical errors when --harmony-sloppy-let is enabled

Stop emitting kSloppyLexical errors when --harmony-sloppy-let is enabled

This changes the error message for code like:

  if (false) let x;

from "Block-scoped declarations (let, const, function, class) not yet supported outside strict mode"
to "Unexpected identifier" (pointing at |x|).

Committed: https://crrev.com/bdf5b39f06ae7af0b7c0972b050d603ed919974a
Cr-Commit-Position: refs/heads/master@{#30834}

[adamk, 2015-09-17 23:06:23]

adamk@chromium.org changed reviewers:
+ littledan@chromium.org

[Dan Ehrenberg, 2015-09-18 02:25:40]

lgtm

[adamk, 2015-09-18 18:17:51]

The CQ bit was checked by adamk@chromium.org

[commit-bot: I haz the power, 2015-09-18 18:17:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1356783002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1356783002/1

[commit-bot: I haz the power, 2015-09-18 18:19:58]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-18 18:20:16]

Patchset 1 (id:??) landed as
https://crrev.com/bdf5b39f06ae7af0b7c0972b050d603ed919974a
Cr-Commit-Position: refs/heads/master@{#30834}