They foreshadow Thisbe's death, as they will be one of the elements that allow her to see that Pyramus is dead.
Explanation:
Snow-white berries were the meeting place where Pyramus and Thisbe would meet and flee in order to live the forbidden love between the two. However, they foreshadow Thysbe's death, as it is through the snow-white berries that she sees the blood of Pyramus and realizes that she cannot remain in this world without his presence. In this case, we can understand that snow-white berries are an example of foreshadoing, as they are advancing plot information, subjectively.
