public Builder addExtension(
          com.google.protobuf.DescriptorProtos.FieldDescriptorProto.Builder builderForValue) {
        if (extensionBuilder_ == null) {
          ensureExtensionIsMutable();
          extension_.add(builderForValue.build());
          onChanged();
        } else {
          extensionBuilder_.addMessage(builderForValue.build());
        }
        return this;
      }