Extracted from https://github.com/facebook/react/issues/12643.

We're currently diff a host node in the complete phase even if it has bailed out in the begin phase. By itself, this just means we're doing a little bit more work than necessary. But it also has some observable effects. In particular, https://github.com/facebook/react/issues/13424 (which by itself would be very rare and probably not worth fixing) is made worse because we end up committing update to inputs from another component.

I have a fix for this in https://github.com/facebook/react/pull/13423. Filing this for posterity.
flushWithoutCommitting(
      expectedFlush: Array<mixed>,
      rootID: string = DEFAULT_ROOT_ID,
    ) {
      const root: any = roots.get(rootID);
      const batch = {
        _defer: true,
        _expirationTime: 1,
        _onComplete: () => {
          root.firstBatch = null;
        },
        _next: null,
      };
      root.firstBatch = batch;
      const actual = ReactNoop.flush();
      expect(actual).toEqual(expectedFlush);
      return (expectedCommit: Array<mixed>) => {
        batch._defer = false;
        NoopRenderer.flushRoot(root, 1);
        expect(yieldedValues).toEqual(expectedCommit);
      };
    },
    getRoot(rootID: string = DEFAULT_ROOT_ID) {
      return roots.get(rootID);
    },
  };
  return ReactNoop;
}
export default createReactNoop;