refactor context to be implicit-downcast safe (#31622)

packages/flutter_tools/lib/src/vmservice.dart

diff --git a/packages/flutter_tools/lib/src/vmservice.dart b/packages/flutter_tools/lib/src/vmservice.dart
index ff5c08f..c5616db 100644
--- a/packages/flutter_tools/lib/src/vmservice.dart
+++ b/packages/flutter_tools/lib/src/vmservice.dart
@@ -82,7 +82,7 @@
       delay *= 2;
   }
 
-  final WebSocketConnector constructor = context[WebSocketConnector] ?? io.WebSocket.connect;
+  final WebSocketConnector constructor = context.get<WebSocketConnector>() ?? io.WebSocket.connect;
   while (socket == null) {
     attempts += 1;
     try {