optimize event emit (fixes #3294)

[ngokevin]

Description:

Emit was potentially creating object in three spots:

• data = {bubbles: !!bubbles, detail: detail}
• data = data || {};
• data.detail = data.detail || {};

This PR makes it to reuse the same object on each emit, except if extraData is provided, in which that customizes the event and we need a separate object.

There is no worry of cross-contamination. The CustomEvent data parameter will pass the object properties along to the event object, but the data parameter objects themselves aren't exposed.

I removed the indirection to the utils.fireEvent (unused, undocumented). It seems its purpose was to create objects if necessary which is no longer needed. It also added detail.target which is not needed because the browser natively has event.target and event.currentTarget. Also made it confusing when event detail objects are reused at application level, the event.detail.target was often incorrect in my experience through contamination.

Changes proposed:

• Get rid of object allocations in .emit() except in the case of extraData.
• Don't allocate empty object for nodeready event, just pass undefined.