Clone on mount

This is the first step towards descriptors. This will start cloning the
component when it's mounted instead of mounting the first instance.

This avoids an issue where a reference to the first instance can hang around
in props. Since a mounted component gets mutated, the descriptor changes.

We don't need to clone the props object itself. Mutating the shallow props
object of a child that's passed into you is already flawed. Those cases need to
use cloneWithProps. A props object is considered shallow frozen after it leaves
the render it was created in.
ReactDOMComponent optimization

Slight optimization to not do unneeded reconciles of DOM components