This is done using desugaring. Before this we had:
result = iterator.next()
with this we instead do:
!%_IsSpecObject(result = iterator.next()) &&
%ThrowIteratorResultNotAnObject(result)
BUG=v8:3916
LOG=N
Review URL: https://codereview.chromium.org/929733003
Cr-Commit-Position: refs/heads/master@{#26805}