As required by the spec.

This is a variant of what I reverted in f47e7224.
It will probably still cause a regression but now it's easier to migrate (parts of)
the current implementation to C++, which is expected to make things faster again.

BUG=chromium:627729,v8:5113

Review-Url: https://codereview.chromium.org/2164923002
Cr-Commit-Position: refs/heads/master@{#37897}