title: How to create instance of class TypeScript?
body: <p>I have the following class:</p>
  
  <pre><code>export declare class FlashMessagesService {
      show: (text?: string, options?: Object) =&gt; void;
      grayOut: (value: boolean) =&gt; void;
  }
  </code></pre>
  
  <p>And I try to create instance of class:</p>
  
  <pre><code> import { FlashMessagesService } from 'angular2-flash-messages';
      let _flashMessagesService = new FlashMessagesService();
  </code></pre>
  
  <p>It invoke an error:</p>
  
  <pre><code>Cannot read property 'show' of undefined
  </code></pre>
  
  <p>I use this <a href="https://github.com/moff/angular2-flash-messages" rel="nofollow noreferrer">library</a></p>