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.
'a class returning undefined',
        async render => {
          class UndefinedComponent extends React.Component {
            render() {
              return undefined;
            }
          }
          await render(<UndefinedComponent />, 1);
        },
        'UndefinedComponent(...): Nothing was returned from render. ' +
          'This usually means a return statement is missing. Or, to ' +
          'render nothing, return null.',
      );
      itThrowsWhenRendering(
        'a function returning an object',
        async render => {
          const ObjectComponent = () => ({x: 123});
          await render(<ObjectComponent />, 1);
        },
        'Objects are not valid as a React child (found: object with keys {x}).' +
          (__DEV__
            ? ' If you meant to render a collection of children, use ' +
              'an array instead.'
            : ''),
      );
      itThrowsWhenRendering(
        'a class returning an object',
        async render => {
          class ObjectComponent extends React.Component {
            render() {