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.
shouldUpdate !== undefined,
          '%s.shouldComponentUpdate(): Returned undefined instead of a ' +
            'boolean value. Make sure to return true or false.',
          getComponentName(workInProgress) || 'Unknown',
        );
      }
      return shouldUpdate;
    }
    if (type.prototype && type.prototype.isPureReactComponent) {
      return (
        !shallowEqual(oldProps, newProps) || !shallowEqual(oldState, newState)
      );
    }
    return true;
  }
  function checkClassInstance(workInProgress: Fiber) {
    const instance = workInProgress.stateNode;
    const type = workInProgress.type;
    if (__DEV__) {
      const name = getComponentName(workInProgress);
      const renderPresent = instance.render;
      if (!renderPresent) {
        if (type.prototype && typeof type.prototype.render === 'function') {
          warning(
            false,
            '%s(...): No `render` method found on the returned component ' +
              'instance: did you accidentally return an object from the constructor?',
            name,
          );
        } else {