Rewrites import.meta expressions into null literals. Builds on top
of- and requires dynamic import parsing to simplify the implementation.

Adds a new --harmony-import-meta flag.

BUG=v8:6693

Change-Id: Iadb7ddf6bad8986bf3ad641dbd3826fe730b5f44
Reviewed-on: https://chromium-review.googlesource.com/702678
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48359}