[release] Add release notes for 2.18.4
[release] Add release notes for 2.18.4

Change-Id: If8ee0291c6baacf8d8c69f73f90ab4602fa58ab1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/266940
Reviewed-by: Michael Thomsen <5977a55c75bb4612004451a56b0b4c22d9d3311d@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