title: Angular 6: Strange "Property xxx is protected and only accessible within class and its subclasses
body: <p>I have a library project that I'm migrating to Angular 6 and Angular CLI 6. </p>
  
  <p>There's an abstract class called <code>NativeFormElementChild</code> that has some private properties: <code>_id</code>, <code>_required</code>, <code>_ariaDescribedBy</code>, and <code>_ariaRequired</code>.</p>
  
  <p>Then I have a directive, <code>InputDirective</code>, that's applied to a <code>&lt;input&gt;</code> as an attribute (adds some extra functionality). <code>InputDirective</code> extends from <code>NativeFormElementChild</code>.</p>
  
  <p>Now when I try to build this in Angular 6, I get errors galore. Everywhere I use my <code>InputDirective</code>, I get four errors:</p>
  
  <ul>
  <li>Property '_id' is protected and only accessible within class 'NativeFormElementChild' and its subclasses.</li>
  <li>Property '_required' is protected and only accessible within class 'NativeFormElementChild' and its subclasses.</li>
  <li>Property '_ariaDescribedBy' is protected and only accessible within class 'NativeFormElementChild' and its subclasses.</li>
  <li>Property '_ariaRequired' is protected and only accessible within class 'NativeFormElementChild' and its subclasses.</li>
  </ul>
  
  <p>I'm not trying to access these properties from anywhere except inside <code>NativeFormElementChild</code>. So I have no idea where these errors are coming from. Using Angular 5, I don't get any of these errors.</p>