This tests and fixes validation failures caused by assignments to
variables holding functions references (which are all considered
immutable). Such references can come from "stdlib" or "foreign".

R=clemensh@chromium.org
TEST=mjsunit/asm/global-imports
BUG=chromium:719382

Change-Id: Ic02be765e0773a6cc74a54e11a09d42ffb683cb8
Reviewed-on: https://chromium-review.googlesource.com/500188Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45189}