Also, polymorphic element stores have a slightly different shape for the array
attached to a vector slot. It's of the form [map, map, handler], where the 2nd
map is either a transition map or undefined (the maps are actually in
WeakCells).

Review URL: https://codereview.chromium.org/1316953003

Cr-Commit-Position: refs/heads/master@{#30432}