Add flutter_adaptive_scaffold package

[johnpryan]

The Adaptive Scaffold (flutter_adaptive_scaffold) package is a collection adaptive widgets from the Material team.

[johnpryan]

Closed in #995

[domesticmouse]

Actually, this isn't shipped to production, so I'm re-opening this issue. This package has been shipped to our hidden testing instance https://dartpad.dev/?channel=dev

Please exercise your suggested package on this instance and confirm here that it works as intended. If you do confirm this as working I'll ship it to stable, beta, master, and old channels on dartpad.dev. If you don't confirm, I won't ship it.