Typo fix for BatchSubscription detach() call
Remove event._super

this._super.method.call(this, ...) only works one superclass away, breaks down
beyond that. It would be fine if this._super.method(...), but that doesn't
preserve overrides.

Removing, forcing events to refer to the appropriate superclass explicitly.
API doc fix removing ref to baseEvent

The base event was killed a while ago in favor of the default event also
serving the role of base event.
Breaking out DOM sub/detach into own methods

The point in the custom event subscription registration or detach where the
native DOM subscription is made or removed was taken out from being inlined to
allow for synthetic events to use the event logic without the native DOM sub
more easily.
Remove type param from CustomEvent constructor

It wasn't used or useful. CustomEvents are sets of behaviors that can be used
to handle multiple events. See DOMEvent, for example. Assigning DOMEvent a type
doesn't make sense.