[bug report] unknown custom element: < El calendar >, < El calendar > component not available

### Element UI version
2.11.1

### OS/Browsers version
win10/chrome 78.0.3904.108

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/pen

### Steps to reproduce
①首先在 main.js 中引入全部element 组件
import ElementUI from 'element-ui';
import 'element-ui/lib/theme-chalk/index.css';
Vue.use(ElementUI, {
size: 'medium',
menuType: 'text'
})
②完成以上步骤后开始在各vue页面中使el 开头的各类组件都没有任何问题,都是好使得.但唯独 不能正常使用.在控制台报错如下:vue.min.js:577 [Vue warn]: Unknown custom element: - did you register the component correctly? For recursive components, make sure to provide the "name" option.我想要的日历也并没有得到渲染.

### What is Expected?
展现出一个日历

### What is actually happening?
Unknown custom element: