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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

'C',
        'Mount [A]',
        'Mount [B]',
        'Mount [C]',
        'Commit root',
        
        'Loading...',
        
        
        'Mount [Loading...]',
      ]);
      expect(ReactNoop.getChildrenAsJSX()).toEqual(
        <React.Fragment>
          <span hidden={true} prop="A" />
          <span hidden={true} prop="C" />
          <span prop="Loading..." />
        </React.Fragment>,
      );
      await advanceTimers(1000);
      expect(ReactNoop.expire(1000)).toEqual(['Promise resolved [B]', 'B']);
      expect(ReactNoop.getChildrenAsJSX()).toEqual(
        <React.Fragment>
          <span prop="A" />
          <span prop="B" />
          <span prop="C" />
        </React.Fragment>,
      );
    });
    it('suspends inside constructor', async () => {
      class AsyncTextInConstructor extends React.Component {