This patch doesn't ship all features of ES2015 variable/scoping
changes, notably omitting the removal of legacy const. I think
function hoisting, let and class in sloppy mode can stand to
themselves as a package, and the legacy const change is much
riskier and more likely to be reverted, so my intention is to
pursue those as a separate, follow-on patch.

R=adamk@chromium.org
BUG=v8:4285,v8:3305
LOG=Y
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_rel_ng;tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1551443002

Cr-Commit-Position: refs/heads/master@{#33133}