[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.