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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

ReactNoop.yield('componentDidMount');
        }
        render() {
          ReactNoop.yield(this.state.text);
          return <span prop={this.state.text} />;
        }
      }
      ReactNoop.renderLegacySyncRoot(
        <Suspense fallback={<Text text="Loading..." />}>
          <AsyncTextInConstructor ms={100} text="Hi" />
        </Suspense>,
      );
      expect(ReactNoop.clearYields()).toEqual([
        'constructor',
        'Suspend! [Hi]',
        'Loading...',
      ]);
      expect(ReactNoop.getChildren()).toEqual([span('Loading...')]);
      await advanceTimers(1000);
      expect(ReactNoop.clearYields()).toEqual([
        'Promise resolved [Hi]',
        'constructor',
        'Hi',
        'componentDidMount',
      ]);
      expect(ReactNoop.getChildren()).toEqual([span('Hi')]);
    });
    it('does not infinite loop if fallback contains lifecycle method', async () => {
      class Fallback extends React.Component {