[release] Add changelog for 2.12.2
[release] Add changelog for 2.12.2

Change-Id: Ia0442151ff6d1ca851e9a7ffc9bd6fd5d1145bc6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/191406
Reviewed-by: Vyacheslav Egorov <72c81f38df725d8f1832acb9b5bb99563fc2f1b9@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk