Removes the legacy analysis options override and fixes all resulting issues. This is a combination of dart fix and manual changes (mostly mechanical, but some small restructuring to address warnings more cleanly, such as creating typed structs from args when they are used repeatedly to avoid repeated casting, or making things that were unnecessarily public private).
One small opportunistic extra cleanup is that the handling of null-safety prerelease versions is removed, as any new plugin would be written null-safe from the start, so we no longer need to allow those versions.
Part of flutter/flutter#76229
Flutter master now creates NNBD code when running 'flutter create', so
the generated pubspec for build_all needs to use a compatible SDK
version. This updates from 2.0.0 to 2.12.0.
Also includes a test for this, which involved setting up tests for the
file, and doing some refactoring to make the command testable. As a
result, this fixes https://github.com/flutter/flutter/issues/61049
(although more test backfill is needed).
