In PrependElementIndicesImpl we sort a FixedArray of indices potentially
containing HeapNumbers. During the string conversion we might trigger a GC.
This in turn might try to read a slot where we previously had a HeapNumber
but the sort sneaked a SMI in there which is not a valid pointer.

BUG=chromium:630561

Review-Url: https://codereview.chromium.org/2173653003
Cr-Commit-Position: refs/heads/master@{#37993}