In wp.Backbone.Subviews, extract subviews with proper Underscore.js functions.

Subviews are stored internally on the Subview manager as an object. The object 
is composed of key-value pairs where the key is a jQuery selector for a view,
and the value is an array of views that matching the selector.

To extract subviews, `_.flatten()` was used to collate the nested arrays of 
views into a single view. However, `_.flatten()` is not intended to be used 
for objects, and this unintended functionality breaks in newer versions of 
Underscore.js. 

Instead, we'll use `_.values()` to extract the arrays of views first, 
and then flatten the array of arrays.

Props adamsilverstein.
See #34350.


git-svn-id: https://develop.svn.wordpress.org/trunk@36305 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/js/wp-backbone.js

@@ -21,7 +21,7 @@ window.wp = window.wp || {};
 		//
 		// Returns an array of all subviews.
 		all: function() {
-			return _.flatten( this._views );
+			return _.flatten( _.values( this._views ) ); 
 		},
 
 		// ### Get a selector's subviews