[BUGFIX] Matches assertion behavior for CPs computing after destroy
[BUGFIX] Matches assertion behavior for CPs computing after destroy

Previously, there was an assertion that threw whenever a CP was
calculated after destroy, _and_ updated its dependent keys (i.e.
updated chains). This meant that CPs without any dependencies would
still run after destroy, and everything would work. The new assertion
always threw if anyone accessed _any_ CP. This changes it to only throw
if the CP has dependencies, matching the old behavior.

tildeio/ember.js,intercom/ember.js,sandstrom/ember.js,sly7-7/ember.js,emberjs/ember.js,elwayman02/ember.js,tildeio/ember.js,stefanpenner/ember.js,mixonic/ember.js,miguelcobain/ember.js,givanse/ember.js,sly7-7/ember.js,elwayman02/ember.js,givanse/ember.js,emberjs/ember.js,knownasilya/ember.js,sly7-7/ember.js,fpauser/ember.js,asakusuma/ember.js,GavinJoyce/ember.js,intercom/ember.js,elwayman02/ember.js,GavinJoyce/ember.js,fpauser/ember.js,asakusuma/ember.js,intercom/ember.js,asakusuma/ember.js,miguelcobain/ember.js,tildeio/ember.js,miguelcobain/ember.js,stefanpenner/ember.js,elwayman02/ember.js,givanse/ember.js,emberjs/ember.js,sandstrom/ember.js,GavinJoyce/ember.js,mfeckie/ember.js,fpauser/ember.js,stefanpenner/ember.js,intercom/ember.js,mixonic/ember.js,miguelcobain/ember.js,knownasilya/ember.js,GavinJoyce/ember.js,mixonic/ember.js,sandstrom/ember.js,asakusuma/ember.js,knownasilya/ember.js,mfeckie/ember.js,fpauser/ember.js,givanse/ember.js,mfeckie/ember.js,mfeckie/ember.js