- 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
