title: Conditional export in ES2015
body: <p>Let's say you're developing a polyfill and don't want to shim a class if it already exists in the browser. How can this be done in ES6? The following is not valid because <code>exports</code> is not a statement:</p>
  
  <pre><code>if (typeof Foo === 'undefined') {
    export class Foo { ... }
  }
  </code></pre>
  
  <p>If the condition above evaluates to <code>false</code>, the importing script should get the browser built-in.</p>