Constructors for speech events

[wicg]

Constructors for speech events

[foolip]

When sending #45 I didn't read the issue description to spot that Edge and Firefox already disagree about whether the argument should be required.

In this case, I would like to align with Firefox, because otherwise the utterance attribute has to be nullable, and the only reason for it would be this constructor. That makes it less clear from looking at the IDL whether it can be null for user-agent-created events.

[foolip]

Yes, it should have a constructor, also raised by @smaug---- before in https://www.w3.org/Bugs/Public/show_bug.cgi?id=28867.

Change should be trivial, but I'm currently converting the spec to Bikeshed, so best to do it after that.

[minorninth]

Interop is good, but what's the use case for constructing one of these?

I suppose it's potentially useful for writing unit tests, but I can't think of anything else.

[smaug----]

Testing, and consistency with other events.