title: subscribe to valueChanges from input FormControl in FormGroup
body: <p>In Angular 4, I'm trying to subscribe to the valueChanges of a FormControl. Neither of the versions below is working. I'm not seeing any errors. The form.value JSON is updating as I type, but the subscription isn't working. </p>
  
  <pre><code>myForm: FormGroup;
  public firstName = new FormControl();
  public lastName = new FormControl();
  
  this.myForm = this.formBuilder.group({ 
        firstName: '',
        lastName: '',
  });
  
  this.myForm.controls.firstName.valueChanges.subscribe(value =&gt; {
        console.log(value);
  });
  
  
  this.myForm.get('firstName').valueChanges.subscribe(value =&gt; {
        console.log('name has changed:', value)
  });
  </code></pre>
  
  <p>Here's a template snippet.</p>
  
  <pre><code>&lt;form #myForm="ngForm"&gt;
  
  &lt;md-input-container&gt;
      &lt;input mdInput name="firstName" [(ngModel)]="firstName" placeholder="enter name"/&gt;
  &lt;/md-input-container&gt;
  </code></pre>
  
  <pre>{{ myForm.value | json }}</pre>