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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

ReactTestRenderer.unstable_yield('Component:' + children);
          return children;
        }
        let renderer;
        SchedulerTracing.unstable_trace('event', mockNow(), () => {
          renderer = ReactTestRenderer.create(<Component>text</Component>, {
            unstable_isConcurrent: true,
          });
        });
        onWorkStarted.mockClear();
        
        throwInOnWorkStarted = true;
        expect(renderer).toFlushAndThrow('Expected error onWorkStarted');
        expect(ReactTestRenderer).toHaveYielded(['Component:text']);
        throwInOnWorkStarted = false;
        expect(onWorkStarted).toHaveBeenCalled();
        
        expect(renderer).toFlushAndYield([]);
        const tree = renderer.toTree();
        expect(tree.type).toBe(Component);
        expect(tree.props.children).toBe('text');
      });
      it('should cover errors thrown in onWorkStopped', () => {
        function Component({children}) {