gistfile1.js

export default function(cb) {
  let counters = 0

  if (typeof Promise === 'undefined') {
    counters++
    require.ensure([], (require) => {
      require('imports?this=>window!es6-promise')
      decrementCounter()
    })
  }

  if (typeof fetch === 'undefined') {
    counters++
    require.ensure([], (require) => {
      require('imports?self=>window!whatwg-fetch')
      decrementCounter()
    })
  }

  maybeFinish()

  function maybeFinish() {
    if (counters === 0) cb()
  }

  function decrementCounter() {
    counters--
    maybeCallback()
  }
}