Migrate to the New Material Buttons (#668)

add_to_app/flutter_module_using_plugin/lib/main.dart

@@ -140,13 +140,13 @@ class Contents extends StatelessWidget {
                 SizedBox(height: 16),
                 Consumer<CounterModel>(
                   builder: (context, model, child) {
-                    return RaisedButton(
+                    return ElevatedButton(
                       onPressed: () => model.increment(),
                       child: Text('Tap me!'),
                     );
                   },
                 ),
-                RaisedButton(
+                ElevatedButton(
                   onPressed: () async {
                     // Use the url_launcher plugin to open the Flutter docs in
                     // a browser.
@@ -159,7 +159,7 @@ class Contents extends StatelessWidget {
                 ),
                 if (showExit) ...[
                   SizedBox(height: 16),
-                  RaisedButton(
+                  ElevatedButton(
                     onPressed: () => SystemNavigator.pop(),
                     child: Text('Exit this screen'),
                   ),