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.
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* Collection set/add/remove/reset methods now return models, not `this`, so they can no longer be chained.
* Options passed to Backbone.View's constructor are no longer attached automatically. wp.Backbone.View now does this automatically.
See [27170] for Backbone 1.1 itself.
props gcorne.
fixes#26799.
Built from https://develop.svn.wordpress.org/trunk@27171
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Eric Lewis
Andrew Nacin
Daryl Koopersmith