* use vendored dart sdk; check the expected version of flutter

* search up directories looking for the flutter root

* have flutter precache print if there's no work to do

* tweak precache

This will help folks run pub get after they upgrade their flutter.git
revision.

* use --no-precompile

* use the flutter bin cache's sdk

Also, resolve our package conflict, since reflectable has been fixed.

Now that we don't require the Dart SDK to be in your path, it's hard to run
./dev/update_packages.dart. Instead, you can now run `flutter update-packages`.

Fixes #1906

This removes a step that can cause trouble.

Fixes #1904