Subclassing FlutterAppDelegate is not required for real-world apps, but
it's what 'flutter create' generates, and applies the default behaviours
that most real-world apps will want.

This patch changes the default `flutter create` command to generate a
HelloServices-style app on iOS. As a consequence, the Atom integration now
fully works with HelloServices.

Cleans up a few style nits in hello_services and adds support for automatic
JSON encoding and decoding to the HostMessages interface.

Currently this service just returns mock data, but it exercises the plumbing.

* add a dev/dartdoc.dart script to generate docs for the packages/ packages

* remove description

* rename readme

* change to using --include-external

* move docs to dev/docs

remove device type specific checks