old_contents: /* @flow */
  import config from '../config';
  import timing from './timing';
  
  export const logSlowReducers = (reducers: Object): Object => {
    Object.keys(reducers).forEach((name: string) => {
      const originalReducer = reducers[name];
      reducers[name] = (state, action) => {
        const startMs = Date.now();
        const result = originalReducer(state, action);
        const endMs = Date.now();
        if (endMs - startMs >= config.slowReducersThreshold) {
          timing.add({
            text: `${action.type} @ ${name}`,
            startMs,
            endMs,
          });
        }
        return result;
      };
    });
    return reducers;
  };
  
new_contents: /* @flow */
  import config from '../config';
  import timing from './timing';
  
  export function logSlowReducers<O: Object>(reducers: O): O {
    Object.keys(reducers).forEach((name: string) => {
      const originalReducer = reducers[name];
      reducers[name] = (state, action) => {
        const startMs = Date.now();
        const result = originalReducer(state, action);
        const endMs = Date.now();
        if (endMs - startMs >= config.slowReducersThreshold) {
          timing.add({
            text: `${action.type} @ ${name}`,
            startMs,
            endMs,
          });
        }
        return result;
      };
    });
    return reducers;
  }
  
subject: Make type less imprecise.