--- a/dom/media/MediaManager.cpp
+++ b/dom/media/MediaManager.cpp
@@ -671,16 +671,57 @@ MediaOperationTask::ReturnCallbackError(
   NS_DispatchToMainThread(do_AddRef(
     new ErrorCallbackRunnable<nsIDOMGetUserMediaSuccessCallback>(onSuccess,
                                                                  mOnFailure,
                                                                  *error,
                                                                  mWindowID)));
 }
 
 /**
+ * This class is only needed since fake tracks are added dynamically.
+ * Instead of refactoring to add them explicitly we let the DOMMediaStream
+ * query us for the source as they become available.
+ * Since they are used only for testing the API surface, we make them very
+ * simple.
+ */
+class FakeTrackSourceGetter : public MediaStreamTrackSourceGetter
+{
+public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(FakeTrackSourceGetter,
+                                           MediaStreamTrackSourceGetter)
+
+  explicit FakeTrackSourceGetter(nsIPrincipal* aPrincipal)
+    : mPrincipal(aPrincipal) {}
+
+  already_AddRefed<dom::MediaStreamTrackSource>
+  GetMediaStreamTrackSource(TrackID aInputTrackID) override
+  {
+    NS_ASSERTION(kAudioTrack != aInputTrackID,
+                 "Only fake tracks should appear dynamically");
+    NS_ASSERTION(kVideoTrack != aInputTrackID,
+                 "Only fake tracks should appear dynamically");
+    return do_AddRef(new BasicUnstoppableTrackSource(mPrincipal));
+  }
+
+protected:
+  virtual ~FakeTrackSourceGetter() {}
+
+  nsCOMPtr<nsIPrincipal> mPrincipal;
+};
+
+NS_IMPL_ADDREF_INHERITED(FakeTrackSourceGetter, MediaStreamTrackSourceGetter)
+NS_IMPL_RELEASE_INHERITED(FakeTrackSourceGetter, MediaStreamTrackSourceGetter)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION_INHERITED(FakeTrackSourceGetter)
+NS_INTERFACE_MAP_END_INHERITING(MediaStreamTrackSourceGetter)
+NS_IMPL_CYCLE_COLLECTION_INHERITED(FakeTrackSourceGetter,
+                                   MediaStreamTrackSourceGetter,
+                                   mPrincipal)
+
+/**
  * Creates a MediaStream, attaches a listener and fires off a success callback
  * to the DOM with the stream. We also pass in the error callback so it can
  * be released correctly.
  *
  * All of this must be done on the main thread!
  *
  * Note that the various GetUserMedia Runnable classes currently allow for
  * two streams.  If we ever need to support getting more than two streams
@@ -865,20 +906,21 @@ public:
       nsCOMPtr<nsIPrincipal> principal;
       if (mPeerIdentity) {
         principal = nsNullPrincipal::Create();
       } else {
         principal = window->GetExtantDoc()->NodePrincipal();
       }
 
       // Normal case, connect the source stream to the track union stream to
-      // avoid us blocking. Pass a null TrackSourceGetter since gUM should never
-      // add tracks dynamically.
+      // avoid us blocking. Pass a simple TrackSourceGetter for potential
+      // fake tracks. Apart from them gUM never adds tracks dynamically.
       domStream =
-        DOMLocalMediaStream::CreateSourceStream(window, msg, nullptr);
+        DOMLocalMediaStream::CreateSourceStream(window, msg,
+                                                new FakeTrackSourceGetter(principal));
 
       if (mAudioDevice) {
         nsString audioDeviceName;
         mAudioDevice->GetName(audioDeviceName);
         const MediaSourceEnum source =
           mAudioDevice->GetSource()->GetMediaSource();
         RefPtr<MediaStreamTrackSource> audioSource =
           new LocalTrackSource(principal, mListener, source, kAudioTrack,