Previously it just had hacks to have NULLs instead of them and pretended to know
nothing about Zone. The hacks provide no real benefit (probably historically
based on some weird misconception about the relationship between Zone and
Isolate), and make it harder for the PreParser to start to know more about
variables and scoping.

BUG=

Review URL: https://codereview.chromium.org/909463003

Cr-Commit-Position: refs/heads/master@{#26494}