Because mockito >=4.1.2 <=5.0.0-nullsafety.7 depends on test_api ^0.2.19-nullsafety and every version of flutter_test from sdk depends on test_api 0.4.8, mockito >=4.1.2 <=5.0.0-nullsafety.7 is incompatible with flutter_test from sdk. So, because my_chat_app depends on both flutter_test from sdk and mockito ^4.1.4, version solving failed. pub get failed (1; So, because my_chat_app depends on both flutter_test from sdk and mockito ^4.1.4, version solving failed.)

[restless226]

pubspec.yaml:

name: my_chat_app
description: A new Flutter application.
publish_to: 'none' # Remove this line if you wish to publish to pub.dev

version: 1.0.0+1

environment:
  sdk: ">=2.7.0 <3.0.0"

dependencies:
  flutter:
    sdk: flutter
  chat:
    path: ./chat
  sqflite: ^2.0.0+3
  cupertino_icons: ^1.0.2

dev_dependencies:
  flutter_test:
    sdk: flutter
  mockito: ^4.1.4
  flutter_lints: ^1.0.0

# The following section is specific to Flutter.
flutter:

chat/pubspec.yaml:

name: chat
description: A new Flutter package project.

environment:
  sdk: ">=2.16.1 <3.0.0"
  flutter: ">=1.17.0"

dependencies:
  flutter:
    sdk: flutter
  rethinkdb_dart: ^2.3.2+6
  encrypt: ^4.1.0

dev_dependencies:
  flutter_test:
    sdk: flutter
  flutter_lints: ^1.0.0

# The following section is specific to Flutter.
flutter:

[restless226]

flutter pub get output:

Because encrypt >=5.0.0-beta.1 depends on crypto ^3.0.0 and rethinkdb_dart 2.3.2+6 depends on crypto ^2.0.6, encrypt >=5.0.0-beta.1 is incompatible with rethinkdb_dart 2.3.2+6.
And because no versions of rethinkdb_dart match >2.3.2+6 <3.0.0, encrypt >=5.0.0-beta.1 is incompatible with rethinkdb_dart ^2.3.2+6.
So, because chat depends on both rethinkdb_dart ^2.3.2+6 and encrypt ^5.0.1, version solving failed.
pub get failed (1; So, because chat depends on both rethinkdb_dart ^2.3.2+6 and encrypt ^5.0.1, version solving failed.)