text: import { inBrowser } from './env'
  // 调用window.performance.mark
  export let mark
  // 
  export let measure
  
  // 如果不是生产环境
  if (process.env.NODE_ENV !== 'production') {
    const perf = inBrowser && window.performance
    /* istanbul ignore if */
    if (
      perf &&
      perf.mark &&
      perf.measure &&
      perf.clearMarks &&
      perf.clearMeasures
    ) {
      mark = tag => perf.mark(tag)
      measure = (name, startTag, endTag) => {
        perf.measure(name, startTag, endTag)
        perf.clearMarks(startTag)
        perf.clearMarks(endTag)
        // perf.clearMeasures(name)
      }
    }
  }
  