code: private static MethodRef getBuilderMethod(Descriptor descriptor) {
      TypeInfo message = messageRuntimeType(descriptor);
      TypeInfo builder = builderRuntimeType(descriptor);
      return MethodRef.createStaticMethod(
              message, new Method("newBuilder", builder.type(), NO_METHOD_ARGS))
          .asNonNullable();
    }
docs: Returns the {@link MethodRef} for the generated newBuilder method.