**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`.

getInitialState: function() {
        return {
          occupation: 'clown',
        };
      },
      render: function() {
        return <span />;
      },
    });
    var instance = <Component />;
    instance = ReactTestUtils.renderIntoDocument(instance);
    expect(instance.state.occupation).toEqual('clown');
  });
  it('renders based on context getInitialState', () => {
    var Foo = createReactClass({
      contextTypes: {
        className: PropTypes.string,
      },
      getInitialState() {
        return {className: this.context.className};
      },
      render() {
        return <span className={this.state.className} />;
      },
    });
    var Outer = createReactClass({
      childContextTypes: {
        className: PropTypes.string,
      },
      getChildContext() {