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Note that this is not a functional change; it just helps authors of idlharness tests identify what's gone wrong if they have incorrect add_objects params.
testharness.js drops the stack for thrown errors and prints a (useless) new stack.

Change-Id: I6dd90c461f0ab58e57978a28e3e5e9eb366a4827
Reviewed-on: https://chromium-review.googlesource.com/1104311
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568803}