add dafrt code blocks (#2521)

Author: pq

lib/src/rules/use_build_context_synchronously.dart

@@ -24,22 +24,22 @@ When a `BuildContext` is used from a `StatefulWidget`, the `mounted` property
 must be checked after an asynchronous gap.
 
 **GOOD:**
-```
+```dart
 void onButtonTapped(BuildContext context) {
   Navigator.of(context).pop();
 }
 ```
 
 **BAD:**
-```
+```dart
 void onButtonTapped(BuildContext context) async {
   await Future.delayed(const Duration(seconds: 1));
   Navigator.of(context).pop();
 }
 ```
 
 **GOOD:**
-```
+```dart
 class _MyWidgetState extends State<MyWidget> {
   ...