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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(onWorkScheduled).toHaveBeenCalled();
        expect(onWorkScheduled.mock.calls[0][0]).toMatchInteractions([
          interactionLowPri,
        ]);
        expect(renderer).toFlushAndYieldThrough(['FirstComponent']);
        expect(onRender).not.toHaveBeenCalled();
        expect(onInteractionTraced).toHaveBeenCalledTimes(1);
        expect(onInteractionTraced).toHaveBeenLastNotifiedOfInteraction(
          interactionLowPri,
        );
        expect(onInteractionScheduledWorkCompleted).not.toHaveBeenCalled();
        expect(getWorkForReactThreads(onWorkStarted)).toHaveLength(1);
        expect(getWorkForReactThreads(onWorkStarted)[0][0]).toMatchInteractions(
          [interactionLowPri],
        );
        expect(getWorkForReactThreads(onWorkStopped)).toHaveLength(0);
        advanceTimeBy(100);
        const interactionHighPri = {
          id: 1,
          name: 'highPri',
          timestamp: mockNow(),
        };
        
        
        renderer.unstable_flushSync(() => {
          SchedulerTracing.unstable_trace(
            interactionHighPri.name,
            mockNow(),
            () => {
              second.setState({count: 1});