E.g.,
{ function lazy_inner(b = somevar) { let somevar; } }

If we don't produce the same scopes, PreParser thinks that the unresolved
variable inside the default parameter resolves into the variable declared inside
the function. Thus, it's not correctly recorded as a free variable.

One part is already done by https://codereview.chromium.org/2638333002 . But at
the laziness boundary, we still produced different scopes.

Unlike previously thought, this is also needed for lazy inner function
correctness, not only for "preparser scope analysis" (ie., skipping inner
functions).

BUG=v8:5938

Change-Id: I047cd43ef16478bb0f18d1f114845e7d1ab8c5f2
Reviewed-on: https://chromium-review.googlesource.com/439345
Commit-Queue: Marja Hölttä <marja@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43044}