The analysis options have gotten behind; this re-syncs to the current state of flutter/flutter. For options that are non-trivial to enable, either because they are non-trivial to fix, or touch a very large number of files, they are locally disabled with clear "LOCAL CHANGE" markers so that it's obvious where we are out of sync. For options that are simple to resolve, they are enabled in the PR.
Part of https://github.com/flutter/flutter/issues/76229
When running a Dart test script for `custom-test`, `pub get` needs to be
run first in order for it to work in a clean environment such as CI.
This will unblock enabling Pigeon's Dart tests in flutter/packages.
