Being able to get the source for your parent components seems useful, and ReactComponentTreeDevtool is best poised to be able to do that.
I'm also not sure it makes sense to have separate DOM-specific `onMountDOMComponent` and `onUpdateDOMComponent` events, so I removed them for now. Even if we want them, their timing seemed sort of arbitrary.
I also made it so DOM devtools can listen to non-DOM events too. Willing to change that if people think it's ugly though.
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