Add quick way to create a new Dart or Flutter snippet with URL

[parlough]

1. Add support for opening DartPad with a brand new Dart or Flutter snippet.
   Perhaps a /dart and /flutter endpoint? Or a query parameter new=flutter?
2. Update flutter.new to redirect to the previous solution chosen in step 1 for Flutter.
3. Redirect dart.new to the previous solution chosen in step 1 for Dart.

These, particularly the Flutter one could use a more detailed sample. Perhaps like an app scaffold of some sort?