Add uses-material-design: true to pubspec.yaml in examples. (#8088)

examples/platform_services/pubspec.yaml

diff --git a/examples/platform_services/pubspec.yaml b/examples/platform_services/pubspec.yaml
index c0cbf0b..a889233 100644
--- a/examples/platform_services/pubspec.yaml
+++ b/examples/platform_services/pubspec.yaml
@@ -7,3 +7,6 @@
 dev_dependencies:
   flutter_test:
     sdk: flutter
+
+flutter:
+  uses-material-design: true