This patch switches sloppy-mode code from legacy const semantics
to ES2015 semantics. It is unknown how much of the web will be
broken by this; likely the patch will have to be reverted before
a branch happens.

BUG=v8:3739
LOG=Y
R=rossberg,adamk

Review URL: https://codereview.chromium.org/1420223003

Cr-Commit-Position: refs/heads/master@{#31525}