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

Bug

**What is the current behavior?**

Calling `this.isMounted()` in `componentWillUnmount` in prior versions would return `true`. Now it returns `false`.

I believe this was untested behavior before, but the new tests that were added may check the wrong value: https://github.com/facebook/react/blob/master/src/isomorphic/classic/class/__tests__/create-react-class-integration-test.js#L417 Changing this line to test for `true` will exhibit the behavior.

The fix would be to defer setting the  `__isMounted` flag to `false` until after all mixins and the `componentWillUnmount` method were called on the component.

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

15.5.x and anything using `create-react-class` is broken. Working correctly in 15.4.x with `React.createClass`.

return {step: 0};
      },
      render() {
        ops.push('Render: ' + this.state.step);
        return <div />;
      },
    });
    var instance = ReactTestUtils.renderIntoDocument(<Component />);
    instance.replaceState({step: 1}, () => {
      ops.push('Callback: ' + instance.state.step);
    });
    expect(ops).toEqual(['Render: 0', 'Render: 1', 'Callback: 1']);
  });
  it('isMounted works', () => {
    spyOn(console, 'error');
    var ops = [];
    var instance;
    var Component = createReactClass({
      displayName: 'MyComponent',
      log(name) {