Style nit: `MESSAGE_METHOD` is a type? Type names are meant to be BigCamlCase (https://docs.google.com/a/google.com/document/d/1ZV70oTNWO00bs0RcMwqr3FvKJXA6smYQ2kZ8Zr8YZnw/edit#heading=h.67jmbtn6e52x)