title: Make Vue not throw error if component is not defined
body: <p>I'm developing a component which have an optional child component.</p>
  
  <pre><code>&lt;template&gt;
    &lt;div&gt;
       &lt;child-component :propA="valueOfA"/&gt; &lt;!-- this child will certainly appear --&gt;
  
       &lt;optional-child-component :propB="valueOfB" /&gt; &lt;!-- this child may not appear --&gt;
    &lt;/div&gt;
  &lt;/template&gt;
  </code></pre>
  
  <p>This is for a library I'm developing. <code>optional-child-component</code> comes from a 3rd-party library that the users of my library may want to use. I won't force them to install 3rd-party libraries if they don't want. The problem is that, currently, if the 3rd-party library is not installed then Vue will throw an error saying that <code>optional-child-component</code> is not defined.</p>
  
  <p>How do I include an optional component in my code given that this component may not be defined/registered?</p>