Migrate example app to Sound Null Safety

example/.flutter-plugins-dependencies

@@ -1 +1 @@
-{"info":"This is a generated file; do not edit or check into version control.","plugins":{"ios":[{"name":"flutter_cast_framework","path":"/Users/gianluca.paradiso/mydev/cast-your-instructions/flutter_cast_framework/","dependencies":[]}],"android":[{"name":"flutter_cast_framework","path":"/Users/gianluca.paradiso/mydev/cast-your-instructions/flutter_cast_framework/","dependencies":[]}],"macos":[],"linux":[],"windows":[],"web":[]},"dependencyGraph":[{"name":"flutter_cast_framework","dependencies":[]}],"date_created":"2021-10-10 18:22:42.460646","version":"2.5.1"}
+{"info":"This is a generated file; do not edit or check into version control.","plugins":{"ios":[{"name":"flutter_cast_framework","path":"/Users/gianluca.paradiso/mydev/cast-your-instructions/flutter_cast_framework/","dependencies":[]}],"android":[{"name":"flutter_cast_framework","path":"/Users/gianluca.paradiso/mydev/cast-your-instructions/flutter_cast_framework/","dependencies":[]}],"macos":[],"linux":[],"windows":[],"web":[]},"dependencyGraph":[{"name":"flutter_cast_framework","dependencies":[]}],"date_created":"2021-10-10 18:24:18.923448","version":"2.5.1"}

example/pubspec.yaml

@@ -4,7 +4,7 @@ version: 0.0.1
 publish_to: 'none'
 
 environment:
-  sdk: ">=2.1.0 <3.0.0"
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   flutter:

example/test/widget_test.dart

@@ -19,7 +19,7 @@ void main() {
     expect(
       find.byWidgetPredicate(
         (Widget widget) => widget is Text &&
-                           widget.data.startsWith('Running on:'),
+                           widget.data!.startsWith('Running on:'),
       ),
       findsOneWidget,
     );