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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

instance.componentWillUnmount();
  stopPhaseTimer();
};
// Capture errors so they don't interrupt unmounting.
function safelyCallComponentWillUnmount(current, instance) {
  if (__DEV__) {
    invokeGuardedCallback(
      null,
      callComponentWillUnmountWithTimer,
      null,
      current,
      instance,
    );
    if (hasCaughtError()) {
      const unmountError = clearCaughtError();
      captureCommitPhaseError(current, unmountError);
    }
  } else {
    try {
      callComponentWillUnmountWithTimer(current, instance);
    } catch (unmountError) {
      captureCommitPhaseError(current, unmountError);
    }
  }
}
function safelyDetachRef(current: Fiber) {
  const ref = current.ref;
  if (ref !== null) {
    if (typeof ref === 'function') {
      if (__DEV__) {