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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

<span prop="Loading (1)" />
          <span prop="Loading (2)" />
          <span prop="Loading (3)" />
        </React.Fragment>,
      );
      await advanceTimers(100);
      expect(ReactNoop.flush()).toEqual([
        'Promise resolved [Step: 2]',
        'Step: 2',
      ]);
      expect(ReactNoop.getChildrenAsJSX()).toEqual(
        <React.Fragment>
          <span prop="Step: 2" />
          <span prop="Sibling" />
        </React.Fragment>,
      );
    });
    it(
      'continues rendering asynchronously even if a promise is captured by ' +
        'a sync boundary (default mode)',
      async () => {
        class UpdatingText extends React.Component {
          state = {text: this.props.initialText};
          render() {
            return this.props.children(this.state.text);
          }
        }
        const text1 = React.createRef(null);
        const text2 = React.createRef(null);
        function App() {