title: Dartlang rxdart - merge different type streams
body: <p>here is my use case:</p>
  
  <p>I have two different <code>Observables</code>:</p>
  
  <pre><code>Observable&lt;MobileAdEvent&gt; adEventStream;
  Observable&lt;Color&gt; colorStream;
  </code></pre>
  
  <p>I need to receive notification when <strong>either</strong> of those two emits an event. <code>zipWith</code> does not work for me, since it waits until both streams has an event to emit. <code>mergeWith</code> also does not seem to work, since it expects both stream to be from the same type.
  Any hints would be greatly appreciated,
  Thanks.</p>