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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

invariant(
        false,
        'This unit of work tag should not have side-effects. This error is ' +
          'likely caused by a bug in React. Please file an issue.',
      );
    }
  }
}
function getHostParentFiber(fiber: Fiber): Fiber {
  let parent = fiber.return;
  while (parent !== null) {
    if (isHostParent(parent)) {
      return parent;
    }
    parent = parent.return;
  }
  invariant(
    false,
    'Expected to find a host parent. This error is likely caused by a bug ' +
      'in React. Please file an issue.',
  );
}
function isHostParent(fiber: Fiber): boolean {
  return (
    fiber.tag === HostComponent ||
    fiber.tag === HostRoot ||
    fiber.tag === HostPortal
  );
}
function getHostSibling(fiber: Fiber): ?Instance {