[Canary] `transitionTo` from Ember.Route#activate nukes parent route template

I encountered this bug in my own app running Canary (`1.13.0-beta.1+canary.5ff3d02e`), and have reproduced it below in a JSBin.

http://jsbin.com/mepasu/edit?html,js,output

In the above JSBin, there are 2 routes (`page` and `page/show`) being explicitly defined, with one being a child of another. Transitioning to one of these routes in the Application `index` route within its `activate` hook causes an issue where the `page` template disappears, leaving behind only its `{{outlet}}`.

If you navigate through the app by clicking on the buttons, you'll notice that you can still transition to `page.index` and `page.show`, but they no longer appear inside of the `page` template.

This issue only seems to affect the `activate` hook – if you uncomment `beforeModel`, `model`, or `afterModel` hooks, they work as expected. There are also no errors being logged/thrown.

cc @rwjblue