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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

componentDidMount() {
        ReactTestRenderer.unstable_yield('Did mount: ' + this.props.label);
      }
      componentDidUpdate() {
        ReactTestRenderer.unstable_yield('Did update: ' + this.props.label);
      }
      render() {
        return <Text text={this.props.label} />;
      }
    }
    async function fakeImport(result) {
      return {default: result};
    }
    const LazyClass = React.lazy(() => fakeImport(Class));
    const root = ReactTestRenderer.create(
      <Suspense fallback={<Text text="Loading..." />}>
        <LazyClass label="Hi" />
      </Suspense>,
    );