title: Angular Material Tabs not working with wrapper component
body: <p>We are developing a corporate component library which should provide Material Designed Angular Components. So the users of this library are not supposed to use e.g. Angular Material directly but rather include some component like "<strong>custom-tabs</strong>".</p>
  
  <p>Using the components of MatTabModule directly works like a charm, whereas when using our custom components the projected content does not show up.</p>
  
  <p>Usage looks very similar to the Angular Material API:</p>
  
  <pre><code>&lt;custom-tabs&gt;
    &lt;custom-tab [label]="labelA"&gt;Content A&lt;/custom-tab&gt;
    &lt;custom-tab [label]="labelB"&gt;Content B&lt;/custom-tab&gt;
    &lt;custom-tab [label]="labelC"&gt;Content C&lt;/custom-tab&gt;
  &lt;/custom-tabs&gt;
  </code></pre>
  
  <p>The custom components try to project the content as follows:</p>
  
  <pre><code>&lt;!-- custom-tabs template --&gt;
  &lt;mat-tab-group&gt;
    &lt;ng-content&gt;&lt;/ng-content&gt;
  &lt;/mat-tab-group&gt;
  
  &lt;!-- custom-tab template --&gt;
  &lt;mat-tab [label]="label"&gt;
    &lt;ng-content&gt;&lt;/ng-content&gt;
  &lt;/mat-tab&gt;
  </code></pre>
  
  <p>Does anyone have an idea how we can get it working?</p>
  
  <p>I provided a <a href="https://stackblitz.com/edit/angular-material2-custom-tabs" rel="noreferrer">StackBlitz</a> where you can see the problem in action.</p>