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.
warning(
            false,
            '%s(...): No `render` method found on the returned component ' +
              'instance: you may have forgotten to define `render`.',
            name,
          );
        }
      }
      const noGetInitialStateOnES6 =
        !instance.getInitialState ||
        instance.getInitialState.isReactClassApproved ||
        instance.state;
      warning(
        noGetInitialStateOnES6,
        'getInitialState was defined on %s, a plain JavaScript class. ' +
          'This is only supported for classes created using React.createClass. ' +
          'Did you mean to define a state property instead?',
        name,
      );
      const noGetDefaultPropsOnES6 =
        !instance.getDefaultProps ||
        instance.getDefaultProps.isReactClassApproved;
      warning(
        noGetDefaultPropsOnES6,
        'getDefaultProps was defined on %s, a plain JavaScript class. ' +
          'This is only supported for classes created using React.createClass. ' +
          'Use a static property to define defaultProps instead.',
        name,
      );
      const noInstancePropTypes = !instance.propTypes;