callHook = function (event, fn, context, arg1, arg2, arg3, arg4, arg5) {
    try {
      fn.call(context, arg1, arg2, arg3, arg4, arg5);
    } catch (e) {
      warning$7(didHookThrowForEvent[event], 'Exception thrown by hook while handling %s: %s', event, e + '\n' + e.stack);
      didHookThrowForEvent[event] = true;
    }
  }
Detect if we can use `window.performance.now()` and gracefully fallback to
`Date.now()` if it doesn't exist. We need to support Firefox < 15 for now
because of Facebook's testing infrastructure.