Attaches the HTTP server to an existing ServerSocket. When the HttpServer is closed, the HttpServer will just detach itself, closing current connections but not closing serverSocket.
factory HttpServer.listenOn(ServerSocket serverSocket) =>
    new _HttpServer.listenOn(serverSocket);
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    https://api.dart.dev/stable/2.5.0/dart-io/HttpServer/HttpServer.listenOn.html