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**Do you want to request a *feature* or report a *bug*?**
Not sure if its a bug or a feature request.

**What is the current behavior?**
The component given as a fallback to Suspense is remounted each time a lazy child resolves.

https://codesandbox.io/s/z6v6x3n1np
This example shows how a fallback component, which counts up, is mounted more than once.

**What is the expected behavior?**
The fallback is mounted once and is only unmounted when all lazy children are resolved.
In the example the counter would run up to 9.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
react@16.6.0

'Suspend! [A]',
        'Loading...',
      ]);
      jest.advanceTimersByTime(1000);
      expect(ReactTestRenderer).toHaveYielded(['Promise resolved [A]', 'A']);
      expect(root).toMatchRenderedOutput('Stateful: 1A');
      root.update(<App text="B" />);
      expect(ReactTestRenderer).toHaveYielded([
        'Stateful: 1',
        'Suspend! [B]',
        'Loading...',
      ]);
      expect(root).toMatchRenderedOutput('Loading...');
      instance.setState({step: 2});
      expect(ReactTestRenderer).toHaveYielded([
        'Stateful: 2',
        'Suspend! [B]',
        'Loading...',
      ]);
      expect(root).toMatchRenderedOutput('Loading...');
      jest.advanceTimersByTime(1000);
      expect(ReactTestRenderer).toHaveYielded([
        'Promise resolved [B]',
        'B',
        'B',
      ]);
      expect(root).toMatchRenderedOutput('Stateful: 2B');
    });
    it('suspends in a class that has componentWillUnmount and is then deleted', () => {
      class AsyncTextWithUnmount extends React.Component {