This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
REACT_STRICT_MODE_TYPE,
  REACT_PROFILER_TYPE,
  REACT_PROVIDER_TYPE,
  REACT_CONTEXT_TYPE,
  REACT_SUSPENSE_TYPE,
  REACT_SUSPENSE_LIST_TYPE,
  REACT_MEMO_TYPE,
  REACT_LAZY_TYPE,
  REACT_SCOPE_TYPE,
  REACT_OFFSCREEN_TYPE,
  REACT_LEGACY_HIDDEN_TYPE,
  REACT_CACHE_TYPE,
} from 'shared/ReactSymbols';
export type {Fiber};
let hasBadMapPolyfill;
if (__DEV__) {
  hasBadMapPolyfill = false;
  try {
    const nonExtensibleObject = Object.preventExtensions({});
    
    new Map([[nonExtensibleObject, null]]);
    new Set([nonExtensibleObject]);
    
  } catch (e) {
    
    hasBadMapPolyfill = true;
  }
}
let debugCounter = 1;
function FiberNode(