A while ago we introduced MutableBigInt in order to enforce this check.

R=jkummerow@chromium.org

Bug: v8:6791
Change-Id: I700ff0b1df854d4f6b8beff6f6c984e11cd07e40
Reviewed-on: https://chromium-review.googlesource.com/881174Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50823}