A script like "{ function foo() {} }" declares a VAR-variable at the
top-level and a LET-variable inside the block.  The LET-variable does
not need to be unconditionally marked as assigned.

BUG=v8:5636

Review-Url: https://codereview.chromium.org/2680443002
Cr-Commit-Position: refs/heads/master@{#42980}