Final answer:
The toast between Carton and Darnay likely foreshadows future events in the story that may involve a change in relationship, sacrifice, or pivotal plot developments.
Step-by-step explanation:
The toast shared between Carton and Darnay likely foreshadows a significant event or revelation in the narrative. Since this is a common literary device, it could suggest a future alliance, a pivotal change in their relationship, or a foretelling of one character stepping into the shoes of another in a sacrificial or heroic act.
As with many such toasts in literature, it may imply a moment of irony or a turning point that will significantly affect the plot's progression. Given the context of a formal setting where such a toast occurs, it may also presage future conflicts or resolutions associated with social status, personal honor, or political intrigue.