[FEATURE propertyBraceExpansion] Brace-expansion only in declarative form.

For performance reasons, property brace expansion is now only supported in
declarative form.  This is very unlikely to affect any users.

So the following works:
```js
var obj = { name: 'Edmure Tully', title: 'Lord Paramount of the Trident' };
Ember.mixin(obj, {
  tridentObserver: function () {
  }.observes('{name,title}')
});
```

While the following does not:
```js
var obj = { name: 'Edmure Tully', title: 'Lord Paramount of the Trident' };
Ember.addObserver(obj, '{name,title}', function () {});
```
Merge pull request #3755 from hjdivad/brace-expansion-declarative-only

[FEATURE propertyBraceExpansion] Brace-expansion only in declarative form.
Store Router.location in container.

This was suggested in #3702, but since that PR targeted the beta branch
we decided not to make significant structural changes.

This enables the location to be looked up from the container (instead of
manually created).