MIDIMessageEventInit and MIDIConnectionEventInit members should be required

[foolip]

http://webaudio.github.io/web-midi-api/#MIDIMessageEvent
http://webaudio.github.io/web-midi-api/#MIDIConnectionEvent

Currently, the init dicts for the event constructors are optional, so new MIDIConnectionEvent('type') should work per spec. But the port attribute then couldn't be anything other than null, and yet it's not nullable.

The way to fix this is to make the init dicts non-optional and to make the members required, e.g.:

[Constructor(DOMString type, MIDIConnectionEventInit eventInitDict)]
interface MIDIConnectionEvent : Event {
    readonly        attribute MIDIPort port;
};

dictionary MIDIConnectionEventInit : EventInit {
             required MIDIPort port;
};

The only other option is to make the port attribute nullable, which would be silly.

[foolip]

Friendly ping :) Blink's IDL is out of sync with the spec here and we'd like to fix it, but ideally not before the spec is fixed.

[cwilso]

I think that #207 has obviated this, but I'm going to keep them both open until I've fixed.

[foolip]

Constructor operations should be unrelated to this, unless one makes drive-by fixes when fixing that of course.

This issue could be summarized as the difference between the IDL in spec and Blink:
https://webaudio.github.io/web-midi-api/#MIDIConnectionEventInit
https://source.chromium.org/chromium/chromium/src/+/master:third_party/blink/renderer/modules/webmidi/midi_connection_event_init.idl;l=8;drc=0aee4434a4dba42a42abaea9bfbc0cd196a63bc1

It looks like the nullability is the only remaining difference, and just making the member nullable in the spec would probably be the most pragmatic here.