fix(orderBy): support string predicates containing non-ident characters

The orderBy filter now allows string predicates passed to the orderBy filter to make use property
name predicates containing non-ident strings, such as spaces or percent signs, or non-latin
characters.

This behaviour requires the predicate string to be double-quoted.

In markup, this might look like so:

```html
<div ng-repeat="item in items | orderBy:'\"Tip %\"'">
...
</div>
```

Or in JS:

```js
var sorted = $filter('orderBy')(array, ['"Tip %"', '-"Subtotal $"'], false);
```

Closes #6143
Closes #6144

src/ng/filter/orderBy.js

@@ -74,6 +74,12 @@ function orderByFilter($parse){
           predicate = predicate.substring(1);
         }
         get = $parse(predicate);
+        if (get.constant) {
+          var key = get();
+          return reverseComparator(function(a,b) {
+            return compare(a[key], b[key]);
+          }, descending);
+        }
       }
       return reverseComparator(function(a,b){
         return compare(get(a),get(b));