修复flutter analyze提示，更新版本至v1.0.0-Beta4.8

frontend/lib/services/chat_service.dart

@@ -1,5 +1,6 @@
 import 'dart:async';
 import 'dart:convert';
+import 'package:flutter/foundation.dart';
 import 'package:web_socket_channel/web_socket_channel.dart';
 
 class ChatMessage {
@@ -38,27 +39,46 @@ class ChatMessage {
 
 class ChatService {
   WebSocketChannel? _channel;
-  final StreamController<ChatMessage> _messageController = StreamController<ChatMessage>.broadcast();
+  final StreamController<ChatMessage> _messageController =
+      StreamController<ChatMessage>.broadcast();
 
   Stream<ChatMessage> get messages => _messageController.stream;
 
   void connect(String baseUrl, String roomId, String username) {
-    final wsUri = Uri.parse(baseUrl).replace(scheme: 'ws', path: '/api/ws/room/$roomId', queryParameters: {'username': username});
+    final wsUri = Uri.parse(baseUrl).replace(
+      scheme: 'ws',
+      path: '/api/ws/room/$roomId',
+      queryParameters: {'username': username},
+    );
     _channel = WebSocketChannel.connect(wsUri);
 
-    _channel!.stream.listen((data) {
-      final json = jsonDecode(data);
-      _messageController.add(ChatMessage.fromJson(json));
-    }, onError: (err) {
-      print("[WS ERROR] $err");
-    }, onDone: () {
-      print("[WS DONE] Connection closed");
-    });
+    _channel!.stream.listen(
+      (data) {
+        final json = jsonDecode(data);
+        _messageController.add(ChatMessage.fromJson(json));
+      },
+      onError: (err) {
+        debugPrint("[WS ERROR] $err");
+      },
+      onDone: () {
+        debugPrint("[WS DONE] Connection closed");
+      },
+    );
   }
 
-  void sendMessage(String content, String username, String roomId, {String type = 'chat'}) {
+  void sendMessage(
+    String content,
+    String username,
+    String roomId, {
+    String type = 'chat',
+  }) {
     if (_channel != null) {
-      final msg = ChatMessage(type: type, username: username, content: content, roomId: roomId);
+      final msg = ChatMessage(
+        type: type,
+        username: username,
+        content: content,
+        roomId: roomId,
+      );
       _channel!.sink.add(jsonEncode(msg.toJson()));
     }
   }