code: public static <ContainingType extends Message, Type>
       GeneratedExtension<ContainingType, Type>
       newFileScopedGeneratedExtension(final Class singularType,
                                       final Message defaultInstance) {
      // For extensions scoped within a file, we rely on the outer class's
      // static initializer to call internalInit() on the extension when the
      // descriptor is available.
      return new GeneratedExtension<ContainingType, Type>(
          null,  // ExtensionDescriptorRetriever is initialized in internalInit();
          singularType,
          defaultInstance);
    }
docs: For use by generated code only.