- [x] Introduce DependencyChecker which can determine if any dependencies have been modified.
- [x] Move the DartDependencyBuilder into a separate file.
- [x] Add unit tests for DartDependencyBuilder.
- [x] Add unit tets for DependencyChecker

Part of #7014 

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.

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