What happened:
- When rewriting in DoParseFunction, the relevant function scope is no longer in
the scope stack.
- The correct scope is given to the PatternRewriter.
- PatternRewriter called to Parser::BuildIteratorCloseForCompletion.
- BuildIteratorCloseForCompletion would just call NewTemporary (which creates
a new temporary in Parser's current scope) instead of using the scope passed to
it and calling NewTemporary on it.
- Normally this went unnoticed, since it doesn't matter that much where the
temporary is.
- But in the lazy arrow func case, the Parser's scope at that point was the
already-resolved outer scope, and a DCHECK detected this problem.

Kudos & thanks to verwaest@ for a debugging session :)

BUG=chromium:761831

Change-Id: I1e8474ce927be0330f4ba4efc0fc08fdcc328809
Reviewed-on: https://chromium-review.googlesource.com/650297
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47877}