Function warnForMissingKey

packages/react-reconciler/src/ReactChildFiber.js:129–133

(
  returnFiber: Fiber,
  workInProgress: Fiber,
  child: mixed,
)

let ownerHasFunctionTypeWarning;
let ownerHasSymbolTypeWarning;
let warnForMissingKey = (
  returnFiber: Fiber,
  workInProgress: Fiber,
  child: mixed,
) => {};

if (__DEV__) {
  didWarnAboutMaps = false;