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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

ReactNoop.expire(2000);
    await advanceTimers(2000);
    expect(ReactNoop.flush()).toEqual([]);
    expect(ReactNoop.getChildren()).toEqual([span('Loading...'), span('Sync')]);
    
    await advanceTimers(1000);
    expect(ReactNoop.flush()).toEqual(['Promise resolved [Async]', 'Async']);
    expect(ReactNoop.getChildren()).toEqual([span('Async'), span('Sync')]);
  });
  it('resolves successfully even if fallback render is pending', async () => {
    ReactNoop.render(
      <Suspense maxDuration={1000} fallback={<Text text="Loading..." />}>
        <AsyncText text="Async" ms={3000} />
      </Suspense>,
    );
    expect(ReactNoop.flushNextYield()).toEqual(['Suspend! [Async]']);
    await advanceTimers(1500);
    expect(ReactNoop.expire(1500)).toEqual([]);
    
    await advanceTimers(2000);
    expect(ReactNoop.clearYields()).toEqual(['Promise resolved [Async]']);
    expect(ReactNoop.flush()).toEqual(['Async']);
    expect(ReactNoop.getChildren()).toEqual([span('Async')]);
  });
  it('throws a helpful error when an update is suspends without a placeholder', () => {
    expect(() => {
      ReactNoop.flushSync(() => ReactNoop.render(<AsyncText text="Async" />));
    }).toThrow(