Work towards https://github.com/flutter/flutter/issues/56591.
I explicitly want an LGTM from @andrewkolos @jmagman @jonahwilliams before merging.
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After this PR, `<Plugin>.isDevDependency` is resolved based on the following logic, IFF:
- The plugin comes from a package _A_ listed in the app's package's `dev_dependencies: ...`
- The package _A_ is not a normal dependency of any transitive non-dev dependency of the app
See [`compute_dev_dependencies_test.dart`](51676093a3/packages/flutter_tools/test/general.shard/compute_dev_dependencies_test.dart) for probably the best specification of this behavior.
We (still) do not write the property to disk (i.e. it never makes it to `.flutter-plugins-dependencies`), so there is no impact to build artifacts at this time; that would come in a follow-up PR (and then follow-up follow-up PRs for the various build systems in both Gradle and Xcode to actually use that value to omit dependencies).
Some tests had to be updated; for the most part it was updating the default `ProcessManager` because a call to `dart pub deps --json` is now made in code that computes what plugins are available, but there should be no change in behavior.
_/cc @jonasfj @sigurdm for FYI only (we talked on an internal thread about this; see https://github.com/dart-lang/sdk/issues/56968)._
_/cc @camsim99 @cbracken @johnmccutchan for visibility on the change._
