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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! [Step: 1]',
        'Sibling',
        'Loading (1)',
        'Loading (2)',
        'Loading (3)',
        'Promise resolved [Step: 1]',
        'Step: 1',
      ]);
      expect(ReactNoop.getChildrenAsJSX()).toEqual(
        <React.Fragment>
          <span prop="Step: 1" />
          <span prop="Sibling" />
        </React.Fragment>,
      );
      
      text.current.setState({step: 2}, () =>
        ReactNoop.yield('Update did commit'),
      );
      
      expect(ReactNoop.flushNextYield()).toEqual(['Suspend! [Step: 2]']);
      expect(ReactNoop.flush()).toEqual([
        'Update did commit',
        
        'Loading (1)',
        'Loading (2)',
        'Loading (3)',
      ]);
      expect(ReactNoop.getChildrenAsJSX()).toEqual(
        <React.Fragment>
          <span hidden={true} prop="Sibling" />