Original issue's description:
> [es6] Better support for built-ins subclassing.
>
> Create proper initial map for original constructor (new.target) instead of doing prototype
> transition on the base constructor's initial map. This approach fixes in-object slack tracking
> for subclass instances.
> This CL also fixes subclassing from String.
>
> BUG=v8:3101, v8:3330
> LOG=Y
>
> Committed: https://crrev.com/cd5f48302a502154a0106d12e3066bd563c6340c
> Cr-Commit-Position: refs/heads/master@{#31680}

It also fixes typed array map smashing done during typed array initialization.

BUG=v8:3101, v8:3330, v8:4419
LOG=Y

Review URL: https://codereview.chromium.org/1413033006

Cr-Commit-Position: refs/heads/master@{#31701}