An identifier may be parsed in an object literal like {let}, but
this was previously left out of lexical name checking. This patch
adds that check to prohibit code like
let {let} = {let: 1}
BUG=v8:4403
LOG=N
R=adamk
Review URL: https://codereview.chromium.org/1401253003
Cr-Commit-Position: refs/heads/master@{#31278}
