Removing Sky from Dart plugin

[sethladd]

Tracking issue. We probably want to remove the "create new Sky project" from the Dart plugin. It's a bit odd that the generic Dart plugin has such specific support for a particular toolkit.

This assumes, of course, that we can build a separate Sky plugin for Atom.

[jlambright]

If creating a Stangehand based project could also be an option, why not leave it?

[devoncarew]

Done! A flutter plugin is published, and we're iteratively moving Flutter UI contributions into it.