[BUGFIX beta] Avoid run.next in app.visit resolve handler.

The prior comment has been pointed to by a few different people in the
FastBoot team meetings so I decided to dig into exactly what the TODO
was talking about. Specifically, why do we need a `run.next` before
resolving the visit promise?

The answer is basically that the `router.handleURL` transition promise
being resolved does not actually guarantee that rendering is completed.
Since the renderer schedules the actual revalidation and rendering into
the `render` queue, we must wait for that to complete before resolving
the promise.

This commit explains that in the inline comments, and changes the
implementation to use `run.schedule('afterRender', ....)` instead of
`run.next` (since resolving "after rendering is complete" is what we
actually care about).

packages/ember-application/lib/system/application-instance.js

@@ -241,20 +241,22 @@ const ApplicationInstance = EngineInstance.extend({
 
     let router = get(this, 'router');
 
-    let handleResolve = () => {
-      // Resolve only after rendering is complete
+    let handleTransitionResolve = () => {
       return new RSVP.Promise((resolve) => {
-        // TODO: why is this necessary? Shouldn't 'actions' queue be enough?
-        // Also, aren't proimses supposed to be async anyway?
-        run.next(null, resolve, this);
+        // Resolve once rendering is completed. `router.handleURL` returns the transition (as a thennable)
+        // which resolves once the transition is completed, but the transition completion only queues up
+        // a scheduled revalidation (into the `render` queue) in the Renderer.
+        //
+        // This uses `run.schedule('afterRender', ....)` to resolve after that rendering has completed.
+        run.schedule('afterRender', null, resolve, this);
       });
     };
 
-    let handleReject = (error) => {
+    let handleTransitionReject = (error) => {
       if (error.error) {
         throw error.error;
       } else if (error.name === 'TransitionAborted' && router.router.activeTransition) {
-        return router.router.activeTransition.then(handleResolve, handleReject);
+        return router.router.activeTransition.then(handleTransitionResolve, handleTransitionReject);
       } else if (error.name === 'TransitionAborted') {
         throw new Error(error.message);
       } else {
@@ -268,7 +270,7 @@ const ApplicationInstance = EngineInstance.extend({
     location.setURL(url);
 
     // getURL returns the set url with the rootURL stripped off
-    return router.handleURL(location.getURL()).then(handleResolve, handleReject);
+    return router.handleURL(location.getURL()).then(handleTransitionResolve, handleTransitionReject);
   }
 });