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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