When the checker was added prohibiting lexical binding called let,
certain error propagation was not implemented properly. This patch
fixes that issue, which fixes error checking for cases such as
  let [let]

BUG=v8:4403
R=adamk
LOG=N

Review URL: https://codereview.chromium.org/1409613003

Cr-Commit-Position: refs/heads/master@{#31289}