The NativeWrapperClasses should be marked base so that none of their subtypes can be implemented.

Implementing subtypes can lead to issues when passing such native wrapper to a native call, as it will try to unwrap a native field that doesn't exist (likely leading to a segfault).

CL: https://dart-review.googlesource.com/c/sdk/+/291761

Flutter has classes which extend NativeWrapperClasses and are mocked in tests.
And Flutter has already opted in to 3.0, so adding the class modifiers to dart:nativewrappers is a breaking change for Flutter.

• Refactor: stop implementing NativeWrapperClasses flutter/flutter#123756

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