The current published core packages don't work with sound null safety in Dart 2.12:

1. They use upper constraints like <2.12.0 which causes confusing pub warnings
2. They use lower constraints like >=2.10.0-78, which pub interprets as not opted into null safety

Repro steps:

1. Update to latest Dart/Flutter master SDK

2. dart create repro

3. cd repro

4. Update pubspec.yaml to have SDK constraint: sdk: '>=2.12.0-0.0 <3.0.0'

5. Update pubspec.yaml to not have a dependency on pedantic

6. dart pub get

7. dart --sound-null-safety run => Runs as expected

8. Edit pubspec.yaml and add a dependency:

   dependencies:
     collection: ^1.15.0-nullsafety
   

9. dart pub get

Issue 1: Upper bound warning

pub get prints:

Resolving dependencies... (1.1s)
Overriding the upper bound Dart SDK constraint to <=2.12.0-3.0.dev for the following packages:

collection

To disable this you can set the PUB_ALLOW_PRERELEASE_SDK system environment variable to `false`, or you can silence this message by setting it to `quiet`.
> collection 1.15.0-nullsafety.4 (was 1.14.13)

issue 2: Sound null safety is disabled:

pub get prints:

The package resolution is not fully migrated to null-safety.

package:collection is not opted into null safety in its pubspec.yaml:
  ╷
8 │   sdk: '>=2.10.0-78 <2.12.0'
  │        ^^^^^^^^^^^^^^^^^^^^^
  ╵

Either downgrade your sdk constraint, or invoke dart/flutter with
`--no-sound-null-safety`.