If we pass in thin-string into a keyed load, the underlying internalized string is used to find the handler. However, the thin string itself was used to interpret the handler. Since the thin string itself isn't unique, this caused existing properties on the prototype chain to not be found in case of dictionary-mode prototypes.

Bug: chromium:731193
Change-Id: Ic98d3789ecf9175e17d9c898ab13231aad59efcc
Reviewed-on: https://chromium-review.googlesource.com/539596Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46005}