**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
checkDepsAreArrayDev(deps);
      return mountCallback(callback, deps);
    },
    useContext<T>(
      context: ReactContext<T>,
      observedBits: void | number | boolean,
    ): T {
      currentHookNameInDev = 'useContext';
      mountHookTypesDev();
      return readContext(context, observedBits);
    },
    useEffect(
      create: () => (() => void) | void,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useEffect';
      mountHookTypesDev();
      checkDepsAreArrayDev(deps);
      return mountEffect(create, deps);
    },
    useImperativeHandle<T>(
      ref: {current: T | null} | ((inst: T | null) => mixed) | null | void,
      create: () => T,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useImperativeHandle';
      mountHookTypesDev();
      checkDepsAreArrayDev(deps);
      return mountImperativeHandle(ref, create, deps);
    },