title: How can I unit test that a PrimeNg p-button is disabled?
body: <p>I'm writing a component that wraps (some) PrimeNg buttons.  I am trying to write a unit test that checks if the button is disabled based on some settings on the outer component.</p>
  
  <p>But, the disabled field is always undefined?  I can see in the Karma/Jasmine screen that it is indeed disabled, so I'm confused as to why I can't get the test to work.</p>
  
  <p>The html template includes:</p>
  
  <pre><code>&lt;p-button id="submit" label="Submit" [disabled]="!enableSubmit""&gt;
  &lt;/p-button&gt;
  </code></pre>
  
  <p>and the test is:</p>
  
  <pre><code>    fit('should disable submit button', done =&gt; {
          component.enableSubmit = false;
  
          fixture.detectChanges();
  
          fixture.whenStable().then(() =&gt; {
              const elem = fixture.debugElement.query(By.css('#submit'));
              const button = elem.nativeElement as Button;
              expect(button.disabled).toEqual(true);
              done();
          });
      });
  </code></pre>
  
  <p>And the result is:</p>
  
  <pre><code>    Expected undefined to equal true.
      Error: Expected undefined to equal true.
  </code></pre>