ionic-framework/packages/angular/scripts/rollup.config.js

import resolve from '@rollup/plugin-node-resolve';

export default {
  input: 'build/es2015/core.js',
  output: [
    {
      file: 'dist/fesm2015.js',
      format: 'es'
    }
  ],
  external: (id) => {
    // anything else is external
    // Windows: C:\xxxxxx\xxx
    const colonPosition = 1;
    return !(id.startsWith('.') || id.startsWith('/') || id.charAt(colonPosition) === ':');
  },
  plugins: [
    resolve({
       mainFields: ['module']
    })
  ]
};