**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
if (deps !== undefined && deps !== null && !Array.isArray(deps)) {
      
      
      warning(
        false,
        '%s received a final argument that is not an array (instead, received `%s`). When ' +
          'specified, the final argument must be an array.',
        currentHookNameInDev,
        typeof deps,
      );
    }
  }
}
function warnOnHookMismatchInDev(currentHookName: HookType) {
  if (__DEV__) {
    const componentName = getComponentName(
      ((currentlyRenderingFiber: any): Fiber).type,
    );
    if (!didWarnAboutMismatchedHooksForComponent.has(componentName)) {
      didWarnAboutMismatchedHooksForComponent.add(componentName);
      if (hookTypesDev !== null) {
        let table = '';
        const secondColumnStart = 30;
        for (let i = 0; i <= ((hookTypesUpdateIndexDev: any): number); i++) {
          const oldHookName = hookTypesDev[i];
          const newHookName =
            i === ((hookTypesUpdateIndexDev: any): number)
              ? currentHookName
              : oldHookName;