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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

// 
          // 
          break;
        }
        previousScheduledRoot = root;
        root = root.nextScheduledRoot;
      }
    }
  }
  nextFlushedRoot = highestPriorityRoot;
  nextFlushedExpirationTime = highestPriorityWork;
}
// TODO: This wrapper exists because many of the older tests (the ones that use
// flushDeferredPri) rely on the number of times `shouldYield` is called. We
// should get rid of it.
let didYield: boolean = false;
function shouldYieldToRenderer() {
  if (didYield) {
    return true;
  }
  if (shouldYield()) {
    didYield = true;
    return true;
  }
  return false;
}
function performAsyncWork() {
  try {
    if (!shouldYieldToRenderer()) {
      //