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`](https://codesandbox.io/s/next-shadowroot-as-root-element-pkktc) (crashes)
- [codesandbox with `react@16.13.1`](https://codesandbox.io/s/16-shadowroot-as-root-element-m74ob) (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 https://github.com/facebook/react/pull/15894
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        )
      ) {
        let isMismatchDueToBadCasing = false;
        if (propertyInfo !== null) {
          
          extraAttributeNames.delete(propertyInfo.attributeName);
          serverValue = getValueForProperty(
            domElement,
            propKey,
            nextProp,
            propertyInfo,
          );
        } else {
          let ownNamespace = parentNamespace;
          if (ownNamespace === HTML_NAMESPACE) {
            ownNamespace = getIntrinsicNamespace(tag);
          }
          if (ownNamespace === HTML_NAMESPACE) {
            
            extraAttributeNames.delete(propKey.toLowerCase());
          } else {
            const standardName = getPossibleStandardName(propKey);
            if (standardName !== null && standardName !== propKey) {
              
              
              
              
              
              isMismatchDueToBadCasing = true;