**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
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