<!--
  Note: if the issue is about documentation or the website, please file it at:
  https://github.com/reactjs/reactjs.org/issues/new
-->

**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

renderer.update(
          <React.unstable_Profiler id="test" onRender={callback}>
            <AdvanceTime />
          </React.unstable_Profiler>,
        );
        expect(callback).toHaveBeenCalledTimes(1);
        [call] = callback.mock.calls;
        expect(call).toHaveLength(enableSchedulerTracing ? 7 : 6);
        expect(call[0]).toBe('test');
        expect(call[1]).toBe('update');
        expect(call[2]).toBe(10); 
        expect(call[3]).toBe(10); 
        expect(call[4]).toBe(35); 
        expect(call[5]).toBe(45); 
        expect(call[6]).toEqual(enableSchedulerTracing ? new Set() : undefined); 
        callback.mockReset();
        advanceTimeBy(20); 
        renderer.update(
          <React.unstable_Profiler id="test" onRender={callback}>
            <AdvanceTime byAmount={4} />
          </React.unstable_Profiler>,
        );
        expect(callback).toHaveBeenCalledTimes(1);
        [call] = callback.mock.calls;
        expect(call).toHaveLength(enableSchedulerTracing ? 7 : 6);
        expect(call[0]).toBe('test');
        expect(call[1]).toBe('update');
        expect(call[2]).toBe(4); 
        expect(call[3]).toBe(4); 
        expect(call[4]).toBe(65);