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Once we enter the isolate, other threads may read the main port as part of attempting to bring all the group members to a reload safepoint. TEST=tsan Change-Id: Ie7ccfd8672c9b923c7e071023a418ddbc58660ae Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/398780 Reviewed-by: Alexander Aprelev <aam@google.com> Commit-Queue: Ryan Macnak <rmacnak@google.com>