[FEATURE propertyBraceExpansion] Ensure __ember_observes__ is set properly.

The refactor in 5d95885 (specifically [this](https://github.com/emberjs/ember.js/commit/5d95885#diff-4482f877489694f981934999cbf3b194L163) section) to allow only the declarative form of expansion caused
the `__ember_observes__` property to be unset when the feature was enabled.

This was identified while fixing the Travis asset publishing and build 
failures in #3863.

@twokul / @rjackson

/cc @hjdivad