feat(utils.getFlattenTree): default to documentElement

[straker]

In investigating various things, we discovered that passing in a subtree of the DOM to getFlattenTree would give bad results when running various APIs. For example, trying to get the accessible name of this structure would return '' and running the label rule would fail:

<div id="foo">lablled</div>
<input  id="context" aria-labelledby="foo">

<script>
var input = document.getElementById('context');
axe.utils.getFlattenTree(document.getElementById('context'));
var a11yName = axe.utils.accessibleText(input);
assert('a11yName === 'foo');  // returns false
</script>

This doesn't happen in a normal axe.run as we always set up the virtual tree to be the documentElement regardless of what node is passed in. This is only an issue with getFlattenTree.

getFlattenTree expects a node to be passed in, but we only can get correct results if documentElement or body is passed in. If any other node is passed in we cannot guarantee correct results. Looking through github for uses, it seems most users are passing in documentElement, but there are some uses of non-documentElement that would cause problems.

To remedy this and not introduce a breaking change, I made passing in a node optional and instead have the code default to documentElement. I also added a note to the docs to warn users of problems passing in a node that is not body or documentElement.

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