<Portal/> and new context API

[kserjey]

Bug Report

Steps

Just try to use new context API.
The problem is caused by using unstable_renderSubtreeIntoContainer function (facebook/react#12493).

Expected Result

Access to context value.

Actual Result

Context value is undefined.

Version

0.81.1

Testcase

https://codesandbox.io/s/3r9nkpk1z5

P.S. Can I help with this issue? I have no experience in contributing, but I'd really like to try...

[welcome]

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[layershifter]

Thanks for the detailed report, will be fixed by #2880.