This makes sure that property lookups on the provided imports object are
non-observable to JavaScript. It allows instantiation failures to fall
back to JavaScript proper without accidentally calling accessors twice.
Also accessors might invalidate previous checks done during linking or
throw exceptions.

R=clemensh@chromium.org
TEST=mjsunit/regress/regress-crbug-719384
BUG=chromium:719384

Change-Id: I3db2672d2a496110f705d02b82878e70cd5d701f
Reviewed-on: https://chromium-review.googlesource.com/509552Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45481}