We have to pessimistically assume that top-level variables will be assigned.
This is because there may be lazily parsed top-level functions, which, for
efficiency, we preparse without variable tracking.

R=adamk@chromium.org, marja@chromium.org
CC=jarin@chromium.org
BUG=v8:5636

Review-Url: https://codereview.chromium.org/2634123002
Cr-Commit-Position: refs/heads/master@{#42398}