title: RxJs Subject.subscribe method not working as expected
body: <p><code>Subject.subscribe</code> method when called outputs the following error: </p>
  
  <pre><code>TypeError: Cannot read property '_subscribe' of undefined
  at BidirectionalSubject._subscribe (Rx.js:10239)
  at BidirectionalSubject._subscribe (Rx.js:10239)
  at BidirectionalSubject.Observable.subscribe (Rx.js:9924)
  at AppComponent.doIt (app.component.ts:32)
  at ChangeDetector_AppComponent_0.handleEventInternal (eval at ChangeDetectorJITGenerator.generate (angular2.dev.js:1), &lt;anonymous&gt;:29:29)
  at ChangeDetector_AppComponent_0.AbstractChangeDetector.handleEvent (angular2.dev.js:8788)
  at AppView.dispatchEvent (angular2.dev.js:9396)
  at AppView.dispatchRenderEvent (angular2.dev.js:9391)
  at DefaultRenderView.dispatchRenderEvent (angular2.dev.js:7819)
  at eventDispatcher (angular2.dev.js:9781)
  </code></pre>
  
  <h2>See Plunkr <a href="https://plnkr.co/edit/UYJJuFeR1ZrWAPaxsA8D" rel="nofollow noreferrer">here</a>.</h2>
  
  <hr>
  
  <p>This is a part of a bigger problem I am facing. <strong>In my actual code, the <code>subscribe</code> method does not throw but it simply does not add an <code>observer</code> to the <code>subject</code>, and hence on all <code>subject.next</code> invocations no one receives the emitted data.</strong></p>
  
  <p>I think the problems are related. In my actual code I am using version <code>5.0.0-beta.12</code> of rxjs. In the plukr though, the <code>rx</code> dependency seems to be coming from angular itself.</p>