When a Mojo bindings method that expects a reply callback is passed a null callback this is currently implemented by waiting for the reply and then simply not calling the callback. This could instead by implemented by treating the message as if it did not expect a reply at all and thus save the overhead of setting up responders on either side of the pipe and serializing and deserializing a message that will be ignored by the caller.

This would be a breaking change to message serialization as the existing message validators expect that all messages with replies have kFlagExpectsResponse set. The StubDispatch classes would also have to be updated to pass a no-op callback to the implementation when no responder is provided. Depending on how this is implemented it could result in a slight increase in the code size for message dispatch.