Dart 2.15 changelog tweaks
Dart 2.15 changelog tweaks

Change-Id: I18efbcddcfe130adfcce8811bf6e1b9c9e70697d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/219440
Auto-Submit: Michael Thomsen <5977a55c75bb4612004451a56b0b4c22d9d3311d@google.com>
Reviewed-by: Leaf Petersen <9bef692239b5f624cec8e2cd5249ab1939a377aa@google.com>
Commit-Queue: Leaf Petersen <9bef692239b5f624cec8e2cd5249ab1939a377aa@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