Follow up from https://github.com/facebook/react/pull/11616.

Here's one I found. These two invariants:

https://github.com/facebook/react/blob/7e7127387ba64f57299eaff8656542cea6fb3a2d/packages/react-reconciler/src/ReactFiberClassComponent.js#L330-L343

are in a function that's behind a DEV block:

https://github.com/facebook/react/blob/7e7127387ba64f57299eaff8656542cea6fb3a2d/packages/react-reconciler/src/ReactFiberClassComponent.js#L447-L449

I don't think this is intentional, but need to verify.

There may be more. I'll use this issue to track while https://github.com/facebook/react/pull/11616 preserves existing behavior.
if (child === undefined) {
    invariant(
      false,
      '%s(...): Nothing was returned from render. This usually means a ' +
        'return statement is missing. Or, to render nothing, ' +
        'return null.',
      getComponentName(type) || 'Component',
    );
  }
}
function resolve(
  child: mixed,
  context: Object,
): {|
  child: mixed,
  context: Object,
|} {
  while (React.isValidElement(child)) {
    
    var element: ReactElement = ((child: any): ReactElement);
    if (__DEV__) {