React.Children support for portals

[gaearon]

I don't think they're currently checked against. We should probably treat them same as elements.

[MatteoVH]

Looking into taking this issue as a first source-code-touching PR, but I'm not sure if it's too much to take at once. If someone who knows what they're doing wants to take over, please let me know so I can look for something else to work on 😄

[gaearon]

See my comment in #11378 (comment). Curious if this is a breaking change.

[gaearon]

Fixed by #11378.