The getVoices method should be asynchronous.

I filed the following bug, does anyone have any thoughts?

https://www.w3.org/Bugs/Public/show_bug.cgi?id=22003

The getVoices method should be asynchronous.

As it is right now, user agents have three choices, all bad:

1. They can pre-load speech synthesis, which slows down application startup
even though most web pages don't speak.
2. They can block until the list of voices is available, which can make the
browser appear sluggish and potentially freeze script execution on that
page for tens of milliseconds.
3. They can return an empty list and update it as soon as the complete list
of voices is available.

I propose that getVoices take a callback as an argument, so you'd use it
like this:

window.speechSynthesis.getVoices(function(voices) {
  for (var i = 0; i < voices.length; i++)
    ...
});

If the callback is unspecified, we could allow it to be used synchronously
- and we should recommend browsers adopt either #2 or #3. Alternatively, we
could just remove support for the synchronous form, and require the
callback.