Previously, we over-approximated Scope::scope_calls_eval_ in
arrow functions: if either the outer scope or the arrow function
parameters had a direct eval call, we marked both scopes as calling
eval. This over-approximation kept getting us into trouble, though,
especially when eager or lazy parsing would disagree about the
"calls eval" bit.

This patch instead tracks eval calls accurately, using a boolean on
Scope::Snapshot that is reset as appropriately depending on whether
a particular AssignmentExpression turned out to be an arrow parameter
list or not.

BUG=chromium:691687

Change-Id: I527dc59b4d32a2797805ff26dc9f70b1311377b2
Reviewed-on: https://chromium-review.googlesource.com/446094
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43499}