This extracts the sample code out from the API doc comments, and places them in separate files on disk, allowing running of the examples locally, testing them, and building of slightly larger examples.

This reverts commit 28db5a0f.

We now use a different approach for ios projects where the developer controls
the Xcode project file. This patch removes the old ".generated" approach in
favor of the new approach.

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.

This example shows how to build a basic app using a custom Xcode project.
Future patches will bring the functionality up to par with hello_android.