Previously, name conflicts between var and let declarations were only
made into exceptions if they were visible at parse-time. This patch adds
runtime checks so that sloppy-mode direct eval can't introduce conflicting
var declarations. The change is implemented by traversing the scope chain
when a direct eval introduces a var declaration to look for conflicting
let declarations, up to the function boundary.

BUG=v8:4454
R=adamk
LOG=Y

Review URL: https://codereview.chromium.org/1382513003

Cr-Commit-Position: refs/heads/master@{#31211}