assess flutter_gallery use_build_context_synchronously breakages

[pq]

A recent fix to use_build_context_synchronously (#3679) is flagging a few places in flutter_gallery that need addressing before we can publish a new linter.

https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8803460924190469121/+/u/analyze_flutter_flutter/stdout

Naively I added some mounted checks but these won't work since the widgets are stateless and don't have a mounted property accessor (flutter/flutter#111321). We could make these diagnostics go away by limiting the lint to only check accesses from StatefulWidgets (indeed the lint docs suggest this) but there's also something fishy going on in the newly flagged calls... I'd love another set of eyes and 2 cents.

Thanks in advance!

/cc @a14n @goderbauer @jacob314

/fyi @raapperez

[pq]

(Marking P1 as this is currently roll-blocking.)

[goderbauer]

The problem also exists for contexts of StatelessWidgets, though. One workaround is to turn the StatelessWidget into a StatefulWidget so you get access to the mounted property. I've also been thinking about adding a property to BuildContext itself so you can check right on it whether it is still valid; discussion for that is at flutter/flutter#111488.

[a14n]

flutter/flutter#111539 fixes the issue.

[pq]

Thanks @a14n!

[pq]

Looks like we've still got a failure:

https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8803023674715529281/+/u/analyze_flutter_gallery/stdout

Analyzing gallery...

   info - lib/pages/demo.dart:198:13 - Do not use BuildContexts across async gaps. - use_build_context_synchronously

[a14n]

flutter/gallery#782 fixes this new issue.

[pq]

Thanks for the quick fix!

[pq]

Thanks!