Fix unmounting performance regression in __DEV__

[gaearon]

This is the same issue as was previously fixed by #7232, but in __DEV__.
Test plan:

1. Grab https://github.com/gaearon/js-framework-benchmark/tree/master/react-v15.3.0
2. npm run build-dev
3. Mount 10K nodes, then “clear”

Before this change: ~6 seconds.
After this change: ~2 seconds.

This seems only relevant for Chrome.

[vjeux]

Quickly skimmed over it, nothing looks crazy. Stamping to unblock

[sophiebits]

Was the try/catch difference noticeable?

[gaearon]

Not very much but it got emitEvent down from 18% to 14% in my testing.

[gaearon]

I'm curious if just having a single handler in hooks and passing the name as an argument could be faster to avoid dynamic checks and lookups.

[sophiebits]

That's actually a very big difference! How much do those numbers vary from run to run?

[gaearon]

To be honest I don’t remember about this particular commit (I’ve been testing many different ways, and some were more flaky than others). I can re-test it again in isolation later if you’d like.

[gaearon]

@zpao This needs an additional fix in #7490