Related pull request:
github
googlesource

1. The intended change in behavior: Add enum type SameSite (link) and public filed sameSite in abstract class Cookie (link) and its default implementation _Cookie (link).
2. The justification/rationale for making the change: This artribute is important for CORS requests, for example, accessing dart backend in Flutter Web. Bacause they runs on different port, browsers don't allow Flutter Web to use cookies of dart backend.
3. The expected impact of this change: all classes implement Cookie must override the filed sameSite.
4. Clear steps for mitigating the change: We can release this change in the beta and dev channels first, so developers can make changes in advance. By the time these changes appeared in the stable channel, developers will have overridden sameSite in their implementation, so their projects will not break.