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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 lowestPriorityPendingInteractiveExpirationTime: ExpirationTime = NoWork;
let hasUnhandledError: boolean = false;
let unhandledError: mixed | null = null;
let isBatchingUpdates: boolean = false;
let isUnbatchingUpdates: boolean = false;
let isBatchingInteractiveUpdates: boolean = false;
let completedBatches: Array<Batch> | null = null;
let originalStartTimeMs: number = now();
let currentRendererTime: ExpirationTime = msToExpirationTime(
  originalStartTimeMs,
);
let currentSchedulerTime: ExpirationTime = currentRendererTime;
// Use these to prevent an infinite loop of nested updates
const NESTED_UPDATE_LIMIT = 50;
let nestedUpdateCount: number = 0;
let lastCommittedRootDuringThisBatch: FiberRoot | null = null;
function recomputeCurrentRendererTime() {
  const currentTimeMs = now() - originalStartTimeMs;
  currentRendererTime = msToExpirationTime(currentTimeMs);
}
function scheduleCallbackWithExpirationTime(
  root: FiberRoot,
  expirationTime: ExpirationTime,
) {
  if (callbackExpirationTime !== NoWork) {
    // 
    if (expirationTime < callbackExpirationTime) {
      // 
      return;
    } else {