[dartdev] Adjust dart compile packages flag description for .packages discontinuation
[dartdev] Adjust dart compile packages flag description for .packages discontinuation

Change-Id: Ie461aa6124308637585f37c26d0f0525236680b6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/240903
Reviewed-by: Ben Konyi <67ffcaedcc3dc3d87dddb9627b72dd41756b31b1@google.com>
Reviewed-by: Jake Macdonald <28dccc213a1e50b0de348b9cb82c0229ef6b65f0@google.com>
Commit-Queue: Ben Konyi <67ffcaedcc3dc3d87dddb9627b72dd41756b31b1@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