<!--
  Note: if the issue is about documentation or the website, please file it at:
  https://github.com/reactjs/reactjs.org/issues/new
-->

**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

componentDidMount() {
          ReactNoop.yield(`Mount [${this.props.text}]`);
        }
        componentDidUpdate() {
          ReactNoop.yield(`Update [${this.props.text}]`);
        }
        render() {
          return <AsyncText {...this.props} />;
        }
      }
      function App() {
        return (
          <Suspense
            maxDuration={1000}
            fallback={<TextWithLifecycle text="Loading..." />}>
            <TextWithLifecycle text="A" />
            <AsyncTextWithLifecycle ms={100} text="B" />
            <TextWithLifecycle text="C" />
          </Suspense>
        );
      }
      ReactNoop.renderLegacySyncRoot(<App />, () =>
        ReactNoop.yield('Commit root'),
      );
      expect(ReactNoop.clearYields()).toEqual([
        'A',
        'Suspend! [B]',