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Write barriers are not eliminated for large arrays, and deferred marking them is expensive. TEST=IsolateSendExitLatency Change-Id: Ifeefb0d86e15d096ae45f8e8ff6aa3d9db4cb9ed Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/373141 Reviewed-by: Alexander Aprelev <aam@google.com> Commit-Queue: Ryan Macnak <rmacnak@google.com>