mdSelect options must be rendered at load time, not just on interaction

[marcysutton]

In working on #3859 and #4914, I came across an issue that has existed for a while (I've mentioned it to @rschmukler a few times):

The options for an mdSelect are not in the DOM at all until you click or use the keyboard to activate it. For screen readers, this means a relationship cannot be established between the select and the options it controls, causing an error in Chrome's A11y Developer Tools:

[Severe] Elements with ARIA roles must ensure required owned elements are present (1)

Here is the part of the ARIA spec this error refers to: http://www.w3.org/TR/wai-aria/roles#mustContain

Furthermore, when the options are rendered, they go into a separate part of the DOM. We can bind a relationship with aria-owns, however, that attribute also requires the matching element to be rendered at load time. When I went to do the work required for #3859, I got even more errors in the Chrome A11y audit stemming from this issue. So the problem isn't going away.

Recommended fix: inject the options on page load so the required ARIA relationships can be established. We can then test aria-owns on mdSelect and decide if the options are accessible enough when rendered in a separate part of the DOM.

For async, we could put in at least one option to start (like a placeholder). I'm not sure how accessible this will be, since it might not refresh the accessibility tree reliably, but it's worth a try.

[calebegg]

Role combobox also technically requires an owned element with role "textbox" (because a combobox normally has an editable text field at the top), though from your investigation in #3859, I think that's not really worth addressing (if NVDA and JAWS both consider the built-in <select> as a "combobox" even though it's not). But it does mean that the audit will continue to fail even if this is fixed.

[marcysutton]

This highlights some of the gaps with native controls and ARIA support. Even though role=combobox is suggested as "a presentation of a select" in the ARIA spec, it doesn't fit the use case where there is no text input, or one that needs to be multiselectable. I think it's fine to use role=listbox in this case, since it will at least tell a screen reader user that they are interacting with some kind of list control.