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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

componentDidMount() {
            advanceTimeBy(5);
            classComponentMounted = true;
          }
          render() {
            advanceTimeBy(2);
            return null;
          }
        }
        ReactTestRenderer.create(
          <React.unstable_Profiler id="test" onRender={callback}>
            <ClassComponent />
          </React.unstable_Profiler>,
        );
        expect(callback).toHaveBeenCalledTimes(1);
      });
    });
  });
  it('should handle interleaved async yields and batched commits', () => {
    jest.resetModules();
    mockDevToolsForTest();
    loadModules({useNoopRenderer: true});
    const Child = ({duration, id}) => {
      ReactNoop.advanceTime(duration);
      ReactNoop.yield(`Child:render:${id}`);
      return null;
    };
    class Parent extends React.Component {