title: Make directive @Input required
body: <p>In AngularJs we could make a directive attribute required. How do we do that in Angular with @Input? The docs don't mention it.</p>
  
  <p>Eg.</p>
  
  <pre><code>@Component({
    selector: 'my-dir',
    template: '&lt;div&gt;&lt;/div&gt;'
  })
  export class MyComponent {
    @Input() a: number; // Make this a required attribute. Throw an exception if it doesn't exist.
    @Input() b: number;
  }
  </code></pre>