Paint (https://api.flutter.dev/flutter/dart-ui/Paint-class.html) is a mutable, framework-specific (i.e. Flutter) "builder" type API.

For example:

void example() {
  Paint paint = Paint();
  if (paintTheTownRed) {
    paint.color = Colors.red;
  }
  /* ... */
}

For historic reasons, Paint is "open" (it can be implemented, extended, or I believe even mixed-in).

Dart added class modifiers in Dart 3.x, which we could utilize:

- class Paint {
+ final class Paint {

Of course this would be a breaking change with no trivial workarounds (other than "stop doing that").

Principles

Here is why I think we should make this change, independent of existing usages:

1. The Paint API was never intended to be user-extendable (the implementation is not versioned, for example)
2. As a mutable object, there is no (good) reason to mock or stub the API - just create a "real" object and mutate it
3. A final class would let us more easily add new methods (or change signatures over time)

For example #142871 becomes trivial if the class was final.

Existing Code

1. Within org:flutter: ~0, well 1 archived repository.
2. Github-wide: ~14. Breakdown below.
3. Internal to Google: I could not find any.

In conclusion: I propose to make this breaking change.