**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
ignorePreviousDependencies = false;
      }
      
      nextCurrentHook = current !== null ? current.memoizedState : null;
      nextWorkInProgressHook = firstWorkInProgressHook;
      currentHook = null;
      workInProgressHook = null;
      componentUpdateQueue = null;
      if (__DEV__) {
        
        hookTypesUpdateIndexDev = -1;
      }
      ReactCurrentDispatcher.current = __DEV__
        ? HooksDispatcherOnUpdateInDEV
        : HooksDispatcherOnUpdate;
      children = Component(props, refOrContext);
    } while (didScheduleRenderPhaseUpdate);
    renderPhaseUpdates = null;
    numberOfReRenders = 0;
  }
  
  
  ReactCurrentDispatcher.current = ContextOnlyDispatcher;
  const renderedWork: Fiber = (currentlyRenderingFiber: any);
  renderedWork.memoizedState = firstWorkInProgressHook;
  renderedWork.expirationTime = remainingExpirationTime;
  renderedWork.updateQueue = (componentUpdateQueue: any);
  renderedWork.effectTag |= sideEffectTag;
  if (__DEV__) {