code: public void addMethodToProto(String methodName) {
      protoClassBuilder.addField(FieldSpec
          .builder(Function.class, methodName, Modifier.PUBLIC)
          .build());
    }
docs: Add a method to the ExposedType proto. This allows referencing them as this.__proto__.myMethod
  to pass them to Vue.js. This way of referencing works both in GWT2 and Closure. The method MUST
  have the @JsMethod annotation for this to work.
  
  @param methodName Name of the method to add to the proto