[release] Move CHANGELOG entries to 2.18
[release] Move CHANGELOG entries to 2.18

Change-Id: I35d887541c8de985b55daed7c732c0d8302d278a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/251542
Reviewed-by: Johnni Winther <0f64ef784d57ffb064b634040f9eeba0d33d0be4@google.com>
Reviewed-by: Alexander Markov <239e774fd29abc1267878f57afe1a8a03e0ca887@google.com>
Reviewed-by: Lasse Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@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