See issue facebook#19701.

This test will fail if we remove propagation, or if we propagate with a root node like fiber.return or fiber.return.return. The additional DEV-only error helps detect a different kind of mistake, like if the thing being passed hasn't actually been encountered on the way up. However, we still leave the actual production loop to check against null so that there is no way we loop forever if the propagation root is wrong.