**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`.

);
    createReactClass({
      displayName: 'NamedComponent',
      componentShouldUpdate: function() {
        return false;
      },
      render: function() {
        return <div />;
      },
    });
    expect(console.error.calls.count()).toBe(2);
    expect(console.error.calls.argsFor(1)[0]).toBe(
      'Warning: NamedComponent has a method called componentShouldUpdate(). Did you ' +
        'mean shouldComponentUpdate()? The name is phrased as a question ' +
        'because the function is expected to return a value.',
    );
  });
  it('should warn when mispelling componentWillReceiveProps', () => {
    spyOn(console, 'error');
    createReactClass({
      componentWillRecieveProps: function() {
        return false;
      },
      render: function() {
        return <div />;
      },
    });
    expect(console.error.calls.count()).toBe(1);
    expect(console.error.calls.argsFor(0)[0]).toBe(
      'Warning: A component has a method called componentWillRecieveProps(). Did you ' +