Message provided to the user when they use the deprecated feature.
The message should explain how to migrate away from the feature if an alternative is available, and when the deprecated feature is expected to be removed.
final String message
© 2012 the Dart project authors
Licensed under the Creative Commons Attribution-ShareAlike License v4.0.
https://api.dart.dev/stable/2.5.0/dart-core/Deprecated/message.html