Taking the slow runtime path for every non-internalized string key
can be avoided by doing optimistic string table lookups: if there
is a matching entry, use that; if there isn't, then no existing
object has a property with that name.
The hashing/internalizing logic is in C++ and called directly.

Review-Url: https://codereview.chromium.org/2811333002
Cr-Commit-Position: refs/heads/master@{#44650}