Fix master channel build (#1900)

- Fixes #1899 - Don't use BuildContext in async.
- Disabled build script for `next-gen-ui-demo` for master channel
because dependency gap does not compile.
- Improve use BuildContext in `place_tracker`, added checks to verify
that the context is still mounted, and avoid passing BuildContext as
parameter.

## 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].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

experimental/federated_plugin/federated_plugin/example/lib/main.dart

@@ -58,6 +58,7 @@ class _HomePageState extends State<HomePage> {
                     batteryLevel = result;
                   });
                 } catch (error) {
+                  if (!context.mounted) return;
                   ScaffoldMessenger.of(context).showSnackBar(
                     SnackBar(
                       backgroundColor: Theme.of(context).primaryColor,