title: Angular 2: Apply Validator.required validation on some condition
body: <p>I have a angular 2 form wherein I have to make a field required on some condition like:</p>
  
  <pre><code>description:  ['', Validators.required]
  </code></pre>
  
  <p>This description field will be required only on some type of a condition like:</p>
  
  <pre><code>if(true){descReq = true};
  </code></pre>
  
  <p>How can I achieve this, please suggest. Thanks in advance!</p>