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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(didMount).toBe(true);
        expect(callback).toHaveBeenCalledTimes(2);
      });
      it('is not invoked until the commit phase', () => {
        const callback = jest.fn();
        const Yield = ({value}) => {
          ReactTestRenderer.unstable_yield(value);
          return null;
        };
        const renderer = ReactTestRenderer.create(
          <React.unstable_Profiler id="test" onRender={callback}>
            <Yield value="first" />
            <Yield value="last" />
          </React.unstable_Profiler>,
          {
            unstable_isConcurrent: true,
          },
        );
        
        expect(renderer).toFlushAndYieldThrough(['first']);
        expect(callback).toHaveBeenCalledTimes(0);
        expect(renderer).toFlushAndYield(['last']);
        expect(callback).toHaveBeenCalledTimes(1);
      });
      it('does not record times for components outside of Profiler tree', () => {
        ReactTestRenderer.create(
          <div>
            <AdvanceTime />
            <AdvanceTime />
            <AdvanceTime />