missingMandatoryExtension top-level constant Null safety
The client is terminating the connection because it expected the server to negotiate one or more extensions, but the server didn't return them in the response message of the WebSocket handshake.
The list of extensions that are needed should appear in the close reason. Note that this status code is not used by the server, because it can fail the WebSocket handshake instead.
const missingMandatoryExtension = 1010;