Most of the remaining widget examples are actually manual tests. This patch
moves them into //dev/manual_tests. A couple are examples of using services,
which I've moved to //examples/layers/services. The remainder are out-dated and
are removed by this patch.
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
