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