**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`.

foo: PropTypes.string,
      },
      contextTypes: {
        foo: PropTypes.string,
      },
      childContextTypes: {
        foo: PropTypes.string,
      },
      render: function() {
        return <div />;
      },
    });
    expect(console.error.calls.count()).toBe(4);
    expect(console.error.calls.argsFor(0)[0]).toBe(
      'createClass(...): `mixins` is now a static property and should ' +
        'be defined inside "statics".',
    );
    expect(console.error.calls.argsFor(1)[0]).toBe(
      'createClass(...): `propTypes` is now a static property and should ' +
        'be defined inside "statics".',
    );
    expect(console.error.calls.argsFor(2)[0]).toBe(
      'createClass(...): `contextTypes` is now a static property and ' +
        'should be defined inside "statics".',
    );
    expect(console.error.calls.argsFor(3)[0]).toBe(
      'createClass(...): `childContextTypes` is now a static property and ' +
        'should be defined inside "statics".',
    );
  });