Dart code now supports the full flow that the C++ code used to: version check,
download, replace app bundle. Bonus: the Dart code is much easier to follow,
thanks to async/await!
This is part 2 of a 3-part change. The first part added new mojom
interfaces, PathService and UpdateService, to the sky_services package.
This adds a new placeholder dart package updater in sky/packages. This is built
into a snapshot and compiled into the Sky engine binary using the same
mechanism as the Dart isolate.
I also added a SkyHeadless class, similar to SkyView, used for running Dart
code without a view.
