[release] Prepare CHANGELOG.md for 2.10 stable
[release] Prepare CHANGELOG.md for 2.10 stable

Change-Id: I89fb513e170f6b989eeec53057367edd2eb79505
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/163584
Reviewed-by: William Hesse <a821cddceae7dc400f272e3cb1a72f400f9fed6d@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