If the eval contains a let, we need to know whether an inner function
refers to the variable to be able to decide its context allocation
status.

The added test needs https://codereview.chromium.org/2435023002/ too
in order to pass.

BUG=v8:5736

Review-Url: https://codereview.chromium.org/2574753002
Cr-Commit-Position: refs/heads/master@{#41723}