Fixups for master CI (#2118)

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- [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.

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platform_channels/lib/src/method_channel_demo.dart

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