[breaking change] Change `SecurityContext` to an `abstract final class`

[brianquinlan]

Change Intent

The intent is to change the dart:io SecurityContext class, used to control the security aspects of a socket connection, from an interface class to an abstract final class.

Justification

A user-implemented SecurityContext cannot be used in place of the dart:io implementation because the Dart TLS implementation depends on C++ data structures found only in its implementation.

Making SecurityContext final also makes it easier for the Dart team to modify in the future e.g. #55679

Impact

This change will break anyone who implements SecurityContext. On GitHub, I only found two such instances, and both were in tests that just raise if a SecurityContext method is called e.g. https://github.com/platform-platform/universal_io/blob/37e2dfcae32156b28477365e9dcf357d583098e5/packages/universal_io/lib/src/driver/driver_impl_vm.dart#L499

Mitigation

Developers must modify (or remove) all classes that implement SecurityContext.

Change Timeline

Remove in Dart 3.5

Associated CLs

No response

[kevmoo]

SGTM!

[itsjustkevin]

@vsmenon @Hixie @leonsenft fresh breaking change request

[leonsenft]

LGTM

[Hixie]

seems reasonable

[a-siva]

@vsmenon can you look at this breaking change and decide.

[brianquinlan]

@vsmenon I'm going to assume that we are good with this ;-)

[itsjustkevin]

@brianquinlan i am marking this breaking change as approved.