Next Gen UI demo

[domesticmouse]

First pass at a Next Generation UI demo app. The UI needs work, feedback gratefully accepted.

Pre-launch Checklist

• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I read the Contributors Guide.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel channel on Discord.

[domesticmouse]

PTAL @craiglabenz

[craiglabenz]

Going to LGTM this because it's very good. My only nit, which I of course consider optional given the LGTM, is to show some sort of progress indicator - which could be as simple as a fraction (e.g., "Step 1 / 10", or what have you).

[domesticmouse]

Going to LGTM this because it's very good. My only nit, which I of course consider optional given the LGTM, is to show some sort of progress indicator - which could be as simple as a fraction (e.g., "Step 1 / 10", or what have you).