void setMockBinaryMessageHandler(String channel, Future<ByteData> handler(ByteData message))

Sets a message handler that intercepts outgoing messages in binary form.

The given callback will replace the currently registered callback (if any). To remove the mock handler, pass null as the handler argument.

Source 

static void setMockBinaryMessageHandler(String channel, Future<ByteData> handler(ByteData message)) {
  if (handler == null)
    _mockHandlers.remove(channel);
  else
    _mockHandlers[channel] = handler;
}