Loading: update code style (#7152)

packages/loading/src/directive.js

 import Vue from 'vue';
 import { addClass, removeClass, getStyle } from 'element-ui/src/utils/dom';
-let Mask = Vue.extend(require('./loading.vue'));
+const Mask = Vue.extend(require('./loading.vue'));
 
 exports.install = Vue => {
   if (Vue.prototype.$isServer) return;
-  let toggleLoading = (el, binding) => {
+  const toggleLoading = (el, binding) => {
     if (binding.value) {
       Vue.nextTick(() => {
         if (binding.modifiers.fullscreen) {
@@ -20,7 +20,7 @@ exports.install = Vue => {
             el.originalPosition = getStyle(document.body, 'position');
 
             ['top', 'left'].forEach(property => {
-              let scroll = property === 'top' ? 'scrollTop' : 'scrollLeft';
+              const scroll = property === 'top' ? 'scrollTop' : 'scrollLeft';
               el.maskStyle[property] = el.getBoundingClientRect()[property] + document.body[scroll] + document.documentElement[scroll] + 'px';
             });
             ['height', 'width'].forEach(property => {
@@ -51,7 +51,7 @@ exports.install = Vue => {
       }
     }
   };
-  let insertDom = (parent, el, binding) => {
+  const insertDom = (parent, el, binding) => {
     if (!el.domVisible && getStyle(el, 'display') !== 'none' && getStyle(el, 'visibility') !== 'hidden') {
       Object.keys(el.maskStyle).forEach(property => {
         el.mask.style[property] = el.maskStyle[property];
@@ -75,7 +75,7 @@ exports.install = Vue => {
 
   Vue.directive('loading', {
     bind: function(el, binding) {
-      let mask = new Mask({
+      const mask = new Mask({
         el: document.createElement('div'),
         data: {
           text: el.getAttribute('element-loading-text'),