Seal or Prevent Extensions on Pooled Events

[gaearon]

I'm going to revert I have reverted #5947 because it's very confusing to see Proxies in the console. The warning is just not worth the debugging impediment.

In the future, we should consider sealing or preventing extensions to pooled events (if we're not getting rid of pooling altogether, that is). But that would be a breaking change.

[gaearon]

I guess that was the original proposal in #5853 anyway

[gaearon]

We don't have pooled events now.