我想创建一个新元素并附加到上的#container部分中connectedCallback。我this.shadowRoot.querySelector("#container")用来选择元素，但总是返回null。如何在阴影根中选择元素？
<!DOCTYPE html>
<html lang="en">
<head>
<script src="https://polygit.org/components/webcomponentsjs/webcomponents-loader.js"></script>
<link rel="import" href="https://polygit.org/components/polymer/polymer-element.html">
</head>
<body>
<dom-module id="dom-element">
<template>
<div id="container">
<p>I'm a DOM element. This is my shadow DOM!</p>
</div>
</template>
<script>
class DomElement extends Polymer.Element {
connectedCallback() {
this.attachShadow({
mode: "open"
})
console.log(this.shadowRoot.querySelector("#container"))
}
static get is() {
return "dom-element";
}
}
customElements.define(DomElement.is, DomElement);
</script>
</dom-module>
<dom-element></dom-element>
</body>
</html>