title: Error: Property timer doesn't exist on type typeof Observable
body: <p>The code is below</p>
  
  <pre><code>import {Component} from 'angular2/core';
  import {Observable} from 'rxjs/Rx';
  
  @Component({
  selector: 'my-app',
  template: 'Ticks (every second) : {{ticks}}'
  })
  export class AppComponent {
     ticks =0;
  
     click(){
        let timer = Observable.timer(2000,1000);
        timer.subscribe(t=&gt;this.ticks = t);
     }
  }
  </code></pre>
  
  <p>But i am getting an error. The error is in the following line: </p>
  
  <pre><code>let timer = Observable.timer(2000,1000);
  </code></pre>
  
  <p>The definition of error is "property timer doesn't exist on type typeof Observable"
  Why am I getting error like that?
  What do you think?</p>