**Do you want to request a *feature* or report a *bug*?**

Bug

**What is the current behavior?**

`isPending` is never set to true when calling `startTransition` within `useEffect`, but it *does* work properly when within a `useLayoutEffect`.

https://codesandbox.io/s/usetransition-useeffect-issues-p1j9s

Here's the correct behavior (accomplished via `useLayoutEffect`):

![good](https://user-images.githubusercontent.com/1500684/68164575-662d7b00-ff1a-11e9-9d02-71d7a22fd5cf.gif)

Here's the incorrect behavior (via `useEffect`):

![bad](https://user-images.githubusercontent.com/1500684/68164540-4c8c3380-ff1a-11e9-9191-d9c87ddecbed.gif)

Note the difference is that the opacity never changes to 0.4 (which is determined based on the `isPending` state).

**What is the expected behavior?**

I expect them to both behave the same (at least as far as the user can observe).

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**

- react@0.0.0-experimental-f6b8d31a7
- react-dom@0.0.0-experimental-f6b8d31a7
numberOfReRenders < RE_RENDER_LIMIT,
    'Too many re-renders. React limits the number of renders to prevent ' +
      'an infinite loop.',
  );
  if (__DEV__) {
    warning(
      typeof arguments[3] !== 'function',
      "State updates from the useState() and useReducer() Hooks don't support the " +
        'second callback argument. To execute a side effect after ' +
        'rendering, declare it in the component body with useEffect().',
    );
  }
  const alternate = fiber.alternate;
  if (
    fiber === currentlyRenderingFiber ||
    (alternate !== null && alternate === currentlyRenderingFiber)
  ) {
    
    
    
    didScheduleRenderPhaseUpdate = true;
    const update: Update<S, A> = {
      expirationTime: renderExpirationTime,
      suspenseConfig: null,
      action,
      eagerReducer: null,
      eagerState: null,
      next: null,
    };
    if (__DEV__) {