Re-enable `platform_channels` CI (#1903)

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] 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

platform_channels/lib/src/method_channel_demo.dart

@@ -46,6 +46,7 @@ class _MethodChannelDemoState extends State<MethodChannelDemo> {
                     final value = await Counter.increment(counterValue: count);
                     setState(() => count = value);
                   } catch (error) {
+                    if (!mounted) return;
                     showErrorMessage(
                       context,
                       (error as PlatformException).message!,
@@ -64,6 +65,7 @@ class _MethodChannelDemoState extends State<MethodChannelDemo> {
                     final value = await Counter.decrement(counterValue: count);
                     setState(() => count = value);
                   } catch (error) {
+                    if (!mounted) return;
                     showErrorMessage(
                       context,
                       (error as PlatformException).message!,

tool/flutter_ci_script_beta.sh

@@ -55,8 +55,7 @@ declare -ar PROJECT_NAMES=(
     # "navigation_and_routing"
     "next_gen_ui_demo"
     "place_tracker"
-    # TODO: https://github.com/flutter/samples/issues/1765
-    # "platform_channels"
+    "platform_channels"
     "platform_design"
     "platform_view_swift"
     "provider_counter"

tool/flutter_ci_script_master.sh

@@ -56,8 +56,7 @@ declare -ar PROJECT_NAMES=(
     # TODO: Dependency 'gap-3.0.0' fails to compile
     # "next_gen_ui_demo"
     "place_tracker"
-    # TODO: https://github.com/flutter/samples/issues/1765
-    # "platform_channels"
+    "platform_channels"
     "platform_design"
     "platform_view_swift"
     "provider_counter"

tool/flutter_ci_script_stable.sh

@@ -49,8 +49,7 @@ declare -ar PROJECT_NAMES=(
     "navigation_and_routing"
     "next_gen_ui_demo"
     "place_tracker"
-    # TODO: https://github.com/flutter/samples/issues/1765
-    # "platform_channels"
+    "platform_channels"
     "platform_design"
     "platform_view_swift"
     "provider_counter"