title: How do I marble-test an observable of void values?
body: <p>Using RXJS's <code>TestScheduler</code>, is there a nice way to marble-test an event-like observable that emits <code>undefined</code> values?</p>
  <p>I am using TypeScript, so type circumvention / monkey-patching is not desirable.</p>
  <pre class="lang-js prettyprint-override"><code>testScheduler.schedule(() =&gt; valueEmittingWork(), 20);
  testScheduler.schedule(() =&gt; valueEmittingWork(), 40);
  
  testScheduler.run(rx =&gt; {
      rx.expectObservable(myObservable).toBe(`20ms ??? 19ms ???`);
      // Above uses the new time progression syntax. What do I put instead of ???
  });
  </code></pre>