Per guidance from legal on current preferred practice, remove "All rights reserved" from all LICENSE files and header blocks, and update the repo tool check accordingly.
Updates links and references to `flutter/plugins` to use `flutter/packages` instead, including making the `repository` pubspec.yaml check stricter in the repo tooling to ensure all packages are pointing to the right place.
Exceptions to the updates are:
- Changelog entries.
- Links to pull requests.
This will re-publish all the moved plugins, thus fixing the current redness of `release` (due to the current versions not being tagged in this repository).
- Updates dependencies to null-safe versions
- Migrates common.dart (which doesn't depend on anything)
- Migrates common_tests.dart and its one dependency, utils.dart
- Adds build_runner for Mockito mock generation
- Adds a new utility methods for getting arguments that handle both the casting and the removal of nullability to address a common problematic pattern while migrating code.
- Converts all files, not just the migrated ones, to those new helpers.
Migrating common.dart and utils.dart should unblock a command-by-command migration to null safety.
Reverts the separate of podspect lints into a step that doesn't do a Flutter upgrade
(https://github.com/flutter/plugins/pull/3700) because without that step we had a
version of Dart too old to run null-safe tooling.
First step of https://github.com/flutter/flutter/issues/81912
