In Dart 3 only sound null safety will be allowed.

This work items describes the gradual roll-out of this change.

Dart 2.19

• Print an info message in the dart tool about the upcoming change:
  dart tool: warn about Dart 3 only allowing sound null safety #49924
• Print an info message in the flutter tool about the upcoming change:
  Update null safety warnings in prep for Dart 3 flutter/flutter#110998).

Dart 3.0 alpha

After the Dart + Flutter development branches (main/master) have branched for Dart 3, we need to make the following changes:

• Update the pub solver to support backwards compatibility with Dart 2.12 and later packages:
  pub client: update solver to support language version 2.12 in Dart 3 pub#3554
• Change the Dart major version number to 3.0:
  https://dart-review.git.corp.google.com/c/sdk/+/271922
• Update templates for 3.0:
  • Dart: Adjust SDK constraint of templates in the dart tool #50635
  • Flutter: Adjust Dart SDK constraint of templates in the flutter tool  flutter/flutter#116583
• Dart analyzer (whether running in an IDE, in dart analyze or flutter analyze: Error on any library that has a language version below 2.12, except when the flag --no-sound-null-safety is passed
  Analyzer doesn't error on legacy libraries #50694
• dart run and dart compile: Error on any library that has a language version below 2.12, except when the flag --no-sound-null-safety is passed
• flutter run and flutter build: Error on any library that has a language version below 2.12, except when the flag --no-sound-null-safety is passed
• dart compile js: ditto
  [dart2js] default to sound null safe mode #50345