(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.
import {
  REACT_FORWARD_REF_TYPE,
  REACT_FRAGMENT_TYPE,
  REACT_STRICT_MODE_TYPE,
  REACT_PROFILER_TYPE,
  REACT_PROVIDER_TYPE,
  REACT_CONTEXT_TYPE,
  REACT_ASYNC_MODE_TYPE,
  REACT_PLACEHOLDER_TYPE,
} from 'shared/ReactSymbols';
let hasBadMapPolyfill;
if (__DEV__) {
  hasBadMapPolyfill = false;
  try {
    const nonExtensibleObject = Object.preventExtensions({});
    const testMap = new Map([[nonExtensibleObject, null]]);
    const testSet = new Set([nonExtensibleObject]);
    
    
    
    testMap.set(0, 0);
    testSet.add(0);
  } catch (e) {
    
    hasBadMapPolyfill = true;
  }
}
export type Fiber = {|