Native `import()` polyfill (Note: can't name it `import` due to reserved word limitations)

importScript.js

Object.defineProperty(window, Symbol.for('registry'), {
  value: new Map(),
});

window.importScript = src => {
  const registry = window[Symbol.for('registry')];

  if (registry.has(src)) {
    return registry.get(src).promise;
  }

  const record = {
    src,
    script: Object.assign(document.createElement('script'), {
      type: 'module',
      innerText: `
        import * as X from '${src}';
        window[Symbol.for('registry')].get('${src}').resolvers[0](X);
      `,
      onload: () => record.script.parentNode.removeChild(record.script),
      onerror: err => {
        registry.get(src).resolvers[1](err);
        record.script.parentNode.removeChild(record.script);
      },
    }),
  };
  record.promise = new Promise((...resolvers) => record.resolvers = resolvers);

  document.head.appendChild(record.script);
  registry.set(src, record);
  return record.promise;
};

// importScript('https://diffhtml.org/es').then(diff => console.log(diff));