error cannot run with sound null safety because the following dependencies don't support null safety code example

Example 1: Error: Cannot run with sound null safety, because the following dependencies don't support null safety:

flutter run --no-sound-null-safety

Example 2: cannot run with sound null safety because the following dependencies don't support null safety

---- Add before imports of file main.dart ----

// @dart=2.9

void main() {
  .........
}