refactor: switch output event event classes to interfaces

[crisbeto]

Converts the various event classes that we emit through @Output into interfaces in order to generate less compiled code and potentially less objects to garbage collect.

[crisbeto]

Caretaker note: we should keep an eye out on how many presubmit failures this causes and determine based on that whether it's still worth the trouble.

[rafaelss95]

FYI this fixes part of #7191.

[jelbourn]

@crisbeto I ran a presubmit on this and found that there are quite a few places where people new their own events in unit tests. Based on that, this would have to be a major change

[rafaelss95]

Any chance to ship this in v6?

[jelbourn]

This isn't going to make it in for 6.0 since it caused too many breakages inside Google. We should bump it to 7.0

[crisbeto]

Closing since this is way out of date.

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