title: Angular 6 error show <mat-panel-title> is not a known element
body: <p>I am trying to use Angular Material in Angular 6 project but getting errors</p>
  
  <ol>
  <li>'mat-panel-title' is not a known element:</li>
  <li>'mat-panel-description' is not a known element:</li>
  <li>'mat-expansion-panel-header' is not a known element:</li>
  <li>'mat-form-field' is not a known element:</li>
  <li>'mat-expansion-panel' is not a known element:</li>
  </ol>
  
  <p>Here is the code</p>
  
  <pre><code>&lt;mat-accordion&gt;
    &lt;mat-expansion-panel&gt;
      &lt;mat-expansion-panel-header&gt;
        &lt;mat-panel-title&gt;
          Personal data
        &lt;/mat-panel-title&gt;
        &lt;mat-panel-description&gt;
          Type your name and age
        &lt;/mat-panel-description&gt;
      &lt;/mat-expansion-panel-header&gt;
  
      &lt;mat-form-field&gt;
        &lt;input matInput placeholder="First name"&gt;
      &lt;/mat-form-field&gt;
  
      &lt;mat-form-field&gt;
        &lt;input matInput placeholder="Age"&gt;
      &lt;/mat-form-field&gt;
    &lt;/mat-expansion-panel&gt;
    &lt;mat-expansion-panel (opened)="panelOpenState = true"
                         (closed)="panelOpenState = false"&gt;
      &lt;mat-expansion-panel-header&gt;
        &lt;mat-panel-title&gt;
          Self aware panel
        &lt;/mat-panel-title&gt;
        &lt;mat-panel-description&gt;
          Currently I am {{panelOpenState ? 'open' : 'closed'}}
        &lt;/mat-panel-description&gt;
      &lt;/mat-expansion-panel-header&gt;
      &lt;p&gt;I'm visible because I am open&lt;/p&gt;
    &lt;/mat-expansion-panel&gt;
  &lt;/mat-accordion&gt;
  </code></pre>
  
  <p>ts file<br>
  <code>panelOpenState = false;</code></p>