title: How to add flatMap using babel 7?
body: <p>After reading the article <a href="https://babeljs.io/blog/2018/07/27/removing-babels-stage-presets" rel="noreferrer">Removing Babel's Stage Presets</a> by <code>babel</code>, I still not fully understand how to add a proposal from, for example, <code>stage-3</code> (<a href="https://github.com/tc39/proposal-flatMap" rel="noreferrer">flatMap</a>) to <code>.babelrc</code>.</p>
  
  <p>As far as I understand, because <code>flatMap</code> can be written in ES5, then I need a polyfill and not a plugin. I installed <code>@babel/polyfill</code> under --save-dev but the browser still tells me that this method doesn't exist. I guess that <code>@babel/polyfill</code> doesn't cover experimental features.</p>