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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

const callback = jest.fn();
        advanceTimeBy(5); 
        ReactTestRenderer.create(
          <React.Fragment>
            <AdvanceTime byAmount={20} />
            <React.unstable_Profiler id="test" onRender={callback}>
              <AdvanceTime byAmount={5} />
            </React.unstable_Profiler>
            <AdvanceTime byAmount={20} />
          </React.Fragment>,
        );
        expect(callback).toHaveBeenCalledTimes(1);
        const [call] = callback.mock.calls;
        expect(call[0]).toBe('test');
        expect(call[2]).toBe(5); 
        expect(call[3]).toBe(5); 
        expect(call[4]).toBe(25); 
        expect(call[5]).toBe(50); 
      });
      it('is not called when blocked by sCU false', () => {
        const callback = jest.fn();
        let instance;
        class Updater extends React.Component {
          state = {};
          render() {
            instance = this;
            return this.props.children;
          }
        }
        class Pure extends React.PureComponent {