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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

ReactTestRenderer.unstable_yield(props.text);
    return props.text;
  }
  function AsyncText(props) {
    const text = props.text;
    try {
      TextResource.read([props.text, props.ms]);
      ReactTestRenderer.unstable_yield(text);
      return text;
    } catch (promise) {
      if (typeof promise.then === 'function') {
        ReactTestRenderer.unstable_yield(`Suspend! [${text}]`);
      } else {
        ReactTestRenderer.unstable_yield(`Error! [${text}]`);
      }
      throw promise;
    }
  }
  it('suspends rendering and continues later', () => {
    function Bar(props) {