function checkKey(element) {
      if (utils.hasDirective(element, 'bind', 'key')) {
        return
      }
      if (element.name === 'template' || element.name === 'slot') {
        for (const child of element.children) {
          if (child.type === 'VElement') {
            checkKey(child)
          }
        }
      } else if (!utils.isCustomComponent(element)) {
        context.report({
          node: element.startTag,
          loc: element.startTag.loc,
          messageId: 'requireKey'
        })
      }
    }
Check the given element about `v-bind:key` attributes.
@param {VElement} element The element node to check.