Defining what should happen if an utterance is changed after passing it to speak()

[TimothyGu]

Consider the following case:

utterance = new SpeechSynthesisUtterance('hello world');
speechSynthesis.speak(utterance)
utterance.text = 'hello world two';

The spec currently says

If changes are made to the SpeechSynthesisUtterance object after calling this method and prior to the corresponding end or error event, it is not defined whether those changes will affect what is spoken, and those changes may cause an error to be returned.

but it's not very satisfactory, as the web platform tries very hard to avoid any possible undefined behavior.

Chrome seems to “snapshot” the actual utterance information at the time of the speak(), which sounds like a good thing to specify.

Also see #29.

[TimothyGu]

On the other hand, the second of the following says "hello world two" in Chrome:

utterance = new SpeechSynthesisUtterance('hello world');
utterance2 = new SpeechSynthesisUtterance('hello world again');
speechSynthesis.speak(utterance)
speechSynthesis.speak(utterance2)
utterance2.text = 'hello world two';

So the internal timing of the SpeechSynthesis queue is actually observable, which seems fairly suboptimal.

cc @foolip