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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

expect(
              ReactTestRenderer.create(
                <React.unstable_Profiler id="label" onRender={jest.fn()} />,
              ).toJSON(),
            ).toMatchSnapshot();
            
            expect(
              ReactTestRenderer.create(
                <div>
                  <React.unstable_Profiler id="label" onRender={jest.fn()} />
                </div>,
              ).toJSON(),
            ).toMatchSnapshot();
          });
          it('should render children', () => {
            const FunctionComponent = ({label}) => <span>{label}</span>;
            const renderer = ReactTestRenderer.create(
              <div>
                <span>outside span</span>
                <React.unstable_Profiler id="label" onRender={jest.fn()}>
                  <span>inside span</span>
                  <FunctionComponent label="function component" />
                </React.unstable_Profiler>
              </div>,
            );
            expect(renderer.toJSON()).toMatchSnapshot();
          });
          it('should support nested Profilers', () => {
            const FunctionComponent = ({label}) => <div>{label}</div>;
            class ClassComponent extends React.Component {