Text nodes within collapsible fieldsets don't collapse

Collapsible fieldsets stick all collapsible elements within a fieldset inside a div for easier collapsing/uncollapsing. This is fine as it goes, but as it only grabs HTML element children of the fieldset, it ignores text nodes that are within the fieldset. This can lead to crazy reordering of content if text nodes exist inside the fieldset, and furthermore, the text nodes don't collapse as the rest of the fieldset collapses, and they are just left there looking awkward on the page. This bug was found using the QUnit testing framework for JavaScript. Want it in core? Help out at the issue. :)