Change Intent

IOOverrides should be marked as base to prevent implementation of the class.

Justification

This class is used to override behavior of certain APIs in dart:io. There's no reason for developers to implement this class and marking it as base will allow for additional overrides to be added to the class without requiring a breaking change (e.g., [ dart:io ] Added override for exit(...) in IOOverrides).

Impact

Code implementing IOOverrides will no longer compile.

Mitigation

PRs should be sent to remove cases where IOOverrides is implemented, where possible. There's only a handful of cases found on GitHub.

Change Timeline

ASAP.

Associated CLs

https://dart-review.googlesource.com/c/sdk/+/389520