rollup.config.js

import babel from 'rollup-plugin-babel';
import common from 'rollup-plugin-commonjs';
import copy from 'rollup-plugin-copy';
import resolve from 'rollup-plugin-node-resolve';

export default {
  input: 'src/index.js',  
  output: {
    file: 'dist/index.js',
    format: 'esm'
  },
  plugins: [
    resolve(),
    common(),
    babel(),
    copy({
      'node_modules/@webcomponents/webcomponentsjs': 'dist/webcomponentsjs',
      verbose: true
    })
  ]
};