Previously, the Parser stored a Scope alongside a RewritableExpression
for each potential destructuring assignment. This Scope was later used
during rewriting to set the correct context for the rewriting. But this
approach failed if a new Scope was inserted into the Scope chain between
the time the assignment was parsed and when it was rewritten.

By storing the Scope directly in RewritableExpression,
ReparentExpressionScopes() is able to appropriately re-scope such
expressions prior to their rewriting.

Bug: chromium:779457
Change-Id: Ieb429a3da841f76d5798610af59da4fccb000652
Reviewed-on: https://chromium-review.googlesource.com/767666
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49368}