title: ion-back-button not showing without defaultHref
body: <p>I'm using angular's own routing <code>this.router.navigate(['/locaties']);</code>. When I navigate forward it still doesn't show up. What am I doing wrong?</p>
  <p>toolbar.component.html</p>
  <pre class="lang-html prettyprint-override"><code>&lt;ion-toolbar color=&quot;primary&quot;&gt;
    &lt;ion-buttons slot=&quot;start&quot;&gt;
      &lt;ion-back-button slot=&quot;start&quot;&gt;&lt;/ion-back-button&gt;
    &lt;/ion-buttons&gt;
  
    &lt;ion-title&gt; {{ _toolbarTitle | async }} &lt;/ion-title&gt;
  
    &lt;img slot=&quot;end&quot; src=&quot;assets/specto_logo.svg&quot; /&gt;
  &lt;/ion-toolbar&gt;
  </code></pre>