The wrappers for imports are not accessible through the code() accessor,
so avoid tracing them for now.

BUG=chromium:844745
R=ahaas@chromium.org

Change-Id: I63fe447091ba3c202e3ffcfddfa645049a0b83cf
Reviewed-on: https://chromium-review.googlesource.com/1068739
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53280}