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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

let hasContext;
  if (isLegacyContextProvider(Component)) {
    hasContext = true;
    pushLegacyContextProvider(workInProgress);
  } else {
    hasContext = false;
  }
  prepareToReadContext(workInProgress, renderExpirationTime);
  constructClassInstance(
    workInProgress,
    Component,
    nextProps,
    renderExpirationTime,
  );
  mountClassInstance(
    workInProgress,
    Component,
    nextProps,
    renderExpirationTime,
  );
  return finishClassComponent(
    null,
    workInProgress,
    Component,
    true,
    hasContext,
    renderExpirationTime,
  );
}
function mountIndeterminateComponent(