code: function observable(options = {}) {
      const observableDecorator = function observableDecorator(elementDescriptor) {
          return getElementDescriptorForProp(
              elementDescriptor.key,
              null,
              ("enumerable" in options) ? options.enumerable : true,
              elementDescriptor.initializer || elementDescriptor.descriptor.get
          );
      };
  
      if (options && options.key) {
          return observableDecorator(options);
      } else {
          return observableDecorator;
      }
  }
docs: Class member decorator that will make that member observable.
  
  @param {Object} [options]
  @param {Boolean} [options.enumerable=true]