for-of should throw if result object is not an object

for-of should throw if result object is not an object

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}