Fix severe perf problems in component tree devtool

[sophiebits]

One of the ReactMultiChildText tests renders 2145 roots (and even more components) and unmounts none of them. Now we don't loop through them all a bunch of times so the test takes 20 seconds instead of 60.

We should clean up instantiateReactComponent somehow so that the onSetDisplayName call isn't produced for the TopLevelWrapper, which should allow us to just store an array of unmountedIDs instead of a hash map so we at least don't have double maps. This change mirrors the old logic though.

Reviewers: @gaearon, @sebmarkbage

[gaearon]

Lgtm. I will be more careful with perf implications of my code on the existing tests in the future!
Ideally we should just set some secret field on both TopLevelWrappers we have, but let’s do this later.

[sophiebits]

Might not be slow just for tests but maybe all dev code! I did a profile and some of the devtools stuff showed up so it might be good to dig in and see what we can optimize. One idea I have is to make each devtool have a consistent shape. That is, in addDevtool you could do

devtools.push({
  onA: devtools.onA,
  onB: devtools.onB,
  onC: devtools.onC,
})

for every possible event. I think JS VMs will generally be happier about that.

[gaearon]

Yeah. It’s a bit hard with ad-hoc profiling and no agreed way of tracking perf regressions.