**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`.

it('should support statics', () => {
    var Component = createReactClass({
      statics: {
        abc: 'def',
        def: 0,
        ghi: null,
        jkl: 'mno',
        pqr: function() {
          return this;
        },
      },
      render: function() {
        return <span />;
      },
    });
    var instance = <Component />;
    instance = ReactTestUtils.renderIntoDocument(instance);
    expect(instance.constructor.abc).toBe('def');
    expect(Component.abc).toBe('def');
    expect(instance.constructor.def).toBe(0);
    expect(Component.def).toBe(0);
    expect(instance.constructor.ghi).toBe(null);
    expect(Component.ghi).toBe(null);
    expect(instance.constructor.jkl).toBe('mno');
    expect(Component.jkl).toBe('mno');
    expect(instance.constructor.pqr()).toBe(Component);
    expect(Component.pqr()).toBe(Component);
  });
  it('should work with object getInitialState() return values', () => {
    var Component = createReactClass({