(Regarding the experimental `React.unstable_Profiler` API and reactjs/rfcs/pull/51.)

PR #13058 changed the behavior of React profiling builds to opt host roots into `ProfileMode` if the React DevTools extension is detected. This was necessary in order to have accurate tree base time measurements for the visual flame graphs.

The Profiler plug-in for DevTools does feature detection to determine if it's enabled by looking at each root and seeing if any of the fibers contain an `actualDuration` attribute,

The problem is that the `actualDuration` attribute will be present on v16.4 fibers, which pre-date PR #13058. As a result, the Profiler UI may enable itself with versions of React that it cannot work correctly with.

Perhaps an easy solution would be to rename one of the `unstable_Profiler`'s fiber attributes prior to the next minor release (e.g. `treeBaseTime` -> `treeBaseDuration`) and change the DevTools Profiler to feature check using this new name instead.
target.stateNode = source.stateNode;
  target.return = source.return;
  target.child = source.child;
  target.sibling = source.sibling;
  target.index = source.index;
  target.ref = source.ref;
  target.pendingProps = source.pendingProps;
  target.memoizedProps = source.memoizedProps;
  target.updateQueue = source.updateQueue;
  target.memoizedState = source.memoizedState;
  target.mode = source.mode;
  target.effectTag = source.effectTag;
  target.nextEffect = source.nextEffect;
  target.firstEffect = source.firstEffect;
  target.lastEffect = source.lastEffect;
  target.expirationTime = source.expirationTime;
  target.alternate = source.alternate;
  if (enableProfilerTimer) {
    target.actualDuration = source.actualDuration;
    target.actualStartTime = source.actualStartTime;
    target.selfBaseTime = source.selfBaseTime;
    target.treeBaseTime = source.treeBaseTime;
  }
  target._debugID = source._debugID;
  target._debugSource = source._debugSource;
  target._debugOwner = source._debugOwner;
  target._debugIsCurrentlyTiming = source._debugIsCurrentlyTiming;
  return target;
}