title: Using the whitelist option with Babel's external-helpers
body: <p>I'm trying to use Rollup with Babel's <code>external-helpers</code>.  It works, but it's dropping a bunch of babel helpers which I don't even need, for example asyncGenerator.</p>
  
  <p>The docs <a href="http://babeljs.io/docs/plugins/external-helpers/" rel="nofollow">show a whitelist option</a> but I can't get it to work</p>
  
  <pre><code>rollup.rollup({
      entry: 'src/buttonDropdown.es6',
      plugins: [
          babel({
              presets: ['react', ['es2015', { modules: false }], 'stage-2'],
              plugins: [['external-helpers', { whitelist: ['asyncGenerator'] }]]
          })
      ]
  })
  </code></pre>
  
  <p>The above has no effect: all Babel helpers are still dropped into my resulting bundle.  </p>
  
  <p>What is the correct way of using this feature, and is there a full list of which helpers' names the whitelist array takes?</p>
  
  <p>Or is there some other Rollup plugin I should be using with Rollup to automatically "tree shake" the babel external helpers.</p>