Migrate `platform_channels` to Material 3 (#1623)

* Migrate platform_channels to Material 3

* fix tests

---------

Co-authored-by: Brett Morgan <brettmorgan@google.com>

platform_channels/lib/main.dart

@@ -25,6 +25,7 @@ class PlatformChannelSample extends StatelessWidget {
         snackBarTheme: SnackBarThemeData(
           backgroundColor: Colors.blue[500],
         ),
+        useMaterial3: true,
       ),
       routerConfig: router(),
     );

platform_channels/lib/src/method_channel_demo.dart

@@ -7,7 +7,7 @@ import 'package:flutter/services.dart';
 import 'package:platform_channels/src/counter_method_channel.dart';
 
 /// The widget demonstrates how to use [MethodChannel] to invoke platform methods.
-/// It has two [ElevatedButton]s to increment and decrement the value of
+/// It has two [FilledButton]s to increment and decrement the value of
 /// [count], and a [Text] widget to display its value.
 class MethodChannelDemo extends StatefulWidget {
   const MethodChannelDemo({super.key});
@@ -38,9 +38,9 @@ class _MethodChannelDemoState extends State<MethodChannelDemo> {
           Row(
             mainAxisAlignment: MainAxisAlignment.spaceEvenly,
             children: [
-              // Whenever users press the ElevatedButton, it invokes
+              // Whenever users press the FilledButton, it invokes
               // Counter.increment method to increment the value of count.
-              ElevatedButton.icon(
+              FilledButton.icon(
                 onPressed: () async {
                   try {
                     final value = await Counter.increment(counterValue: count);
@@ -56,9 +56,9 @@ class _MethodChannelDemoState extends State<MethodChannelDemo> {
                 label: const Text('Increment'),
               ),
 
-              // Whenever users press the ElevatedButton, it invokes
+              // Whenever users press the FilledButton, it invokes
               // Counter.decrement method to decrement the value of count.
-              ElevatedButton.icon(
+              FilledButton.icon(
                 onPressed: () async {
                   try {
                     final value = await Counter.decrement(counterValue: count);

platform_channels/lib/src/platform_image_demo.dart

@@ -60,7 +60,7 @@ class _PlatformImageDemoState extends State<PlatformImageDemo> {
             const SizedBox(
               height: 16,
             ),
-            ElevatedButton(
+            FilledButton(
               onPressed: imageData != null
                   ? null
                   : () {

platform_channels/test/src/platform_image_demo_test.dart

@@ -29,7 +29,7 @@ void main() {
       expect(find.byType(Image), findsNothing);
 
       // Tap on ElevatedButton to get Image.
-      await tester.tap(find.byType(ElevatedButton));
+      await tester.tap(find.byType(FilledButton));
       await tester.pumpAndSettle();
 
       expect(find.byType(Placeholder), findsNothing);