React version: 0.0.0-f77c7b9d7

Steps To Reproduce

1. Mount a component with even listeners in a ShadowRoot

Link to code example:

• codesandbox with react@next (crashes)
• codesandbox with react@16.13.1 (mounts without errors/warnings)

The current behavior

Throws with

ensureListeningTo(): received a container that was not an element node. This is likely a bug in React.

The expected behavior

1. throw with a descriptive error that ShadowRoot is no longer supported as a root element

or

2. continue to allow ShadowRoot as a root element

No preference here. This was just from an old regression test we had.

Will be fixed by #15894