title: angular8 es5 son call parent method call this.method is undefined
body: <pre><code>export class SonComponent extends ParentComponent {
  
   public say() {
       console.info('hello！my parent is ' + super.getName());
   }
  }
  </code></pre>
  
  <pre><code>export class ParentComponent {
    public getName() {
      return this.getFirstName() + this.getLastName(); // &lt;--- exception, core.js:4002 ERROR TypeError: this.getFirstName is not a function
    }
    public getFirstName() {
      return 'wu';
    }
    public getLastName() {
      return 'victor';
    }
  </code></pre>
  
  <p>exception, core.js:4002 ERROR TypeError: this.getFirstName is not a function</p>
  
  <p>why?</p>