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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

advanceTimeBy = amount => {
    currentTime += amount;
  };
  AdvanceTime = class extends React.Component {
    static defaultProps = {
      byAmount: 10,
      shouldComponentUpdate: true,
    };
    shouldComponentUpdate(nextProps) {
      return nextProps.shouldComponentUpdate;
    }
    render() {
      
      advanceTimeBy(this.props.byAmount);
      return this.props.children || null;
    }
  };
  resourcePromise = null;
  function yieldForRenderer(value) {
    if (ReactNoop) {
      ReactNoop.yield(value);
    } else {
      ReactTestRenderer.unstable_yield(value);
    }
  }
  TextResource = ReactCache.unstable_createResource(([text, ms = 0]) => {
    resourcePromise = new Promise((resolve, reject) =>
      setTimeout(() => {
        yieldForRenderer(`Promise resolved [${text}]`);
        resolve(text);