A loader for both `.mjs` and `.m.js` extension.

es.mjs

import _url from 'url';

const builtins = new Set(
  Object.keys(process.binding('natives')).filter(str =>
    /^(?!(?:internal|node|v8)\/)/.test(str))
);

const esmIfExplicit = url => {
  const eof = url.slice(-5);
  return {
    url,
    format: eof === '.m.js' || eof.slice(1) === '.mjs' ?
            'esm' : 'dynamic'
  };
};

export const resolve = (specifier, base) =>
  builtins.has(specifier) ?
    {url: specifier, format: 'builtin'} :
    esmIfExplicit(new _url.URL(specifier, base).href;