docs: private static MethodRef getPutMethod(FieldDescriptor descriptor) {
      Preconditions.checkState(descriptor.isMapField());
      List<FieldDescriptor> mapFields = descriptor.getMessageType().getFields();
      TypeInfo builder = builderRuntimeType(descriptor.getContainingType());
      return MethodRef.createInstanceMethod(
          builder,
          new Method(
              "put" + getFieldName(descriptor, true),
              builder.type(),
              new Type[] {getRuntimeType(mapFields.get(0)), getRuntimeType(mapFields.get(1))}));
    }