<!--
  Note: if the issue is about documentation or the website, please file it at:
  https://github.com/reactjs/reactjs.org/issues/new
-->

**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

mountClassInstance,
  resumeMountClassInstance,
  updateClassInstance,
} from './ReactFiberClassComponent';
import {readLazyComponentType} from './ReactFiberLazyComponent';
import {
  resolveLazyComponentTag,
  createFiberFromTypeAndProps,
  createFiberFromFragment,
  createWorkInProgress,
  isSimpleFunctionComponent,
} from './ReactFiber';
const ReactCurrentOwner = ReactSharedInternals.ReactCurrentOwner;
let didWarnAboutBadClass;
let didWarnAboutContextTypeOnFunctionComponent;
let didWarnAboutGetDerivedStateOnFunctionComponent;
let didWarnAboutFunctionRefs;
if (__DEV__) {
  didWarnAboutBadClass = {};
  didWarnAboutContextTypeOnFunctionComponent = {};
  didWarnAboutGetDerivedStateOnFunctionComponent = {};
  didWarnAboutFunctionRefs = {};
}
export function reconcileChildren(
  current: Fiber | null,
  workInProgress: Fiber,
  nextChildren: any,
  renderExpirationTime: ExpirationTime,
) {
  if (current === null) {