Update CHANGELOG.md for 2.17.6 patch release
Update CHANGELOG.md for 2.17.6 patch release

This is a patch release that fixes:

- Improve code completion for Flutter (issue [#49054][]).
- Fix crash on ARM (issue [#106510][]).
- Fix compiler crash with Finalizable parameters (issue [#49402][]).

[#49054]: https://github.com/dart-lang/sdk/issues/49054
[#106510]: https://github.com/flutter/flutter/issues/106510
[#49402]: https://github.com/dart-lang/sdk/issues/49402

Change-Id: I9a814603d9793b75b764fd0f384e89017e773c50
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/251109
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>
Commit-Queue: 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