title: Property 'unsubscribe' does not exist on type 'Observable<DataSnapshot>'
body: <p>Typescript(tslint in atom editor) is giving me a typescript error, but i cannot figure out how to set the correct type.</p>
  
  <p>Error message:
  <a href="https://i.stack.imgur.com/VU3ee.png" rel="noreferrer"><img src="https://i.stack.imgur.com/VU3ee.png" alt="enter image description here"></a></p>
  
  <p>chat component:</p>
  
  <pre><code>  private _chatObserver: Observable&lt;firebase.database.DataSnapshot&gt;
  
    otherMethod () {
          this._chatObserver = this._chat.observe(alarmId)
          this._chatObserver.subscribe(
            (messageSnap: firebase.database.DataSnapshot) =&gt; {
              this.messages.push(messageSnap.val())
            },
            error =&gt; {throw error})
      }
  
      ionViewDidLeave() {
         this._chatObserver.unsubscribe() 
      }
  </code></pre>
  
  <p>_chat provider:</p>
  
  <pre><code>  public observe (alarmId){
      let messagesRef = this._ref.child(`alarms/${alarmId}/messages`)
  
      const observable = Observable.create(observer =&gt; {
        messagesRef.on('child_added',(messageSnap) =&gt; {
              observer.next(messageSnap)
          },
          (error) =&gt; observer.error(error)
        )
        return () =&gt; {
          messagesRef.off('value')
        };
      });
  
      return observable
    }
  </code></pre>