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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

ReactNoop.yield(`Update [${this.props.text}]`);
      }
      componentWillUnmount() {
        ReactNoop.yield(`Unmount [${this.props.text}]`);
      }
      render() {
        const text = this.props.text;
        const ms = this.props.ms;
        try {
          TextResource.read([text, ms]);
          ReactNoop.yield(text);
          return <span prop={text} />;
        } catch (promise) {
          if (typeof promise.then === 'function') {
            ReactNoop.yield(`Suspend! [${text}]`);
          } else {
            ReactNoop.yield(`Error! [${text}]`);
          }
          throw promise;
        }
      }
    }
    function App() {
      return (
        <Suspense
          maxDuration={1000}
          fallback={<TextWithLifecycle text="Loading..." />}>
          <TextWithLifecycle text="A" />
          <AsyncTextWithLifecycle ms={100} text="B" />