code: public static String getJsExtensionName(FieldDescriptor desc) {
      Descriptor scope = desc.getExtensionScope();
      if (scope != null) {
        return calculateQualifiedJsName(scope) + "." + computeJsExtensionName(desc);
      }
      return getJsPackage(desc.getFile()) + "." + computeJsExtensionName(desc);
    }
docs: Returns the JS name of the extension, suitable for passing to getExtension().