[improvement] adjust utils

packages/dialog/index.js

@@ -1,9 +1,13 @@
 import Vue from 'vue';
 import VanDialog from './Dialog';
-import { isServer, isInDocument } from '../utils';
+import { isServer } from '../utils';
 
 let instance;
 
+function isInDocument(element) {
+  return document.body.contains(element);
+}
+
 function initInstance() {
   if (instance) {
     instance.$destroy();