For example let and class should only be allowed inside function/block/script.

We have to continue to support const in statements in sloppy mode for backwards compatibility.

BUG=3831
LOG=Y
R=dslomov@chromium.org, adamk

Review URL: https://codereview.chromium.org/869293002

Cr-Commit-Position: refs/heads/master@{#26337}