Remove dispatchEvent emulation.
IE9+ support means built-in dispatchEvent.
Feature test for MutationObserver was throwing
Event#stopImmediatePropagation is a no-op
Instead of over-riding stopPropagation(), etc in event listeners,
do this once on the Event.prototype
Add Event.prototype._stopPropagation as a back-door for
the MutationObserver emulation using MutationEvent
The previous implementations of no-op stopPropagation() were being called
when external code wasn't expecting it (MutationEvent, PopStateEvent).
Need to limit it to events that DOMSprockets is managing -
hence Binding.managedEvents[] and Binding.manageEvent():

Remove _stopPropagation() as a back-door.
Nullify stopPropagation() in a listener attached to window that handles the capture phase of events. This is as early as possible.