<!--
  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

ReactTestRenderer.unstable_yield(
            `Update [${this.props.text}:${this.state.step}]`,
          );
        }
        componentWillUnmount() {
          ReactTestRenderer.unstable_yield(
            `Unmount [${this.props.text}:${this.state.step}]`,
          );
        }
        render() {
          instance = this;
          const text = `${this.props.text}:${this.state.step}`;
          const ms = this.props.ms;
          try {
            TextResource.read([text, ms]);
            ReactTestRenderer.unstable_yield(text);
            return text;
          } catch (promise) {
            if (typeof promise.then === 'function') {
              ReactTestRenderer.unstable_yield(`Suspend! [${text}]`);
            } else {
              ReactTestRenderer.unstable_yield(`Error! [${text}]`);
            }
            throw promise;
          }
        }
      }
      function App() {