I am creating simple webcomponent now I want to import css , I found a method using adpotedstylesheet.

Here is how I import it my webcomponet

import sheet from './styles/swal.css' assert { type: 'css' };

class Demo extends HTMLElement{

   constructor() {
    this.attachShadow({ mode: "open" });
    this.shadowRoot.appendChild(Demo.content.cloneNode(true));
    document.adoptedStyleSheets = [sheet];
    this.shadowRoot.adoptedStyleSheets = [sheet];
   }
}

window.customElements.define("demo-component", Demo);

and here is my rollup settup for bundling my component

import summary from "rollup-plugin-summary";
import { terser } from "rollup-plugin-terser";
import resolve from "@rollup/plugin-node-resolve";
import replace from "@rollup/plugin-replace";
import postcss from "rollup-plugin-postcss";
import { eslint } from "rollup-plugin-eslint";
import babel from "rollup-plugin-babel";
import { uglify } from "rollup-plugin-uglify";
import commonjs from 'rollup-plugin-commonjs';

export default {
  input: "index.js",
  output: {
    file: "dist/index.js",
    format: "esm",
  },
  onwarn(warning) {
    if (warning.code !== "THIS_IS_UNDEFINED") {
      console.error(`(!) ${warning.message}`);
    }
  },
  plugins: [
    postcss({
      plugins: [],
      extensions: [".css"],
    }),
    resolve({
      jsnext: true,
      main: true,
      browser: true,
    }),
    commonjs(),
    eslint({
      exclude: ["src/styles/**"],
    }),
    babel({
      exclude: "node_modules/**",
    }),
    terser({
      ecma: 2017,
      module: true,
      warnings: true,
      mangle: {
        properties: {
          regex: /^__/,
        },
      },
    }),
    summary(),
    replace({
      "Reflect.decorate": "undefined",
      preventAssignment: true,
      ENV: JSON.stringify(process.env.NODE_ENV || "development"),
    }),
    process.env.NODE_ENV === "production" && uglify(),
  ],
};

Now when i run npm run buil I get the following error.

[!] (plugin commonjs) SyntaxError: Unexpected token (3:38)`

What am I doing wrong here ???