DartPad appears on mobile Safari

[johnpryan]

I'm not sure if we want to show DartPad on mobile devices. It still works, but isn't quite as usable.

[parlough]

The new DartPad UI has a more usable, adaptive experience on mobile devices, so I'm going to close this for now in favor of more specific issue on the DartPad repository around the mobile experience.

I also would like to see much of the uses of DartPad on dart.dev replaced with a more lightweight experience as well. Tracking that possibility in dart-lang/dart-pad#2702.