// `isModernFlag` is only available during build since it is resolved by `vite:build-import-analysis`
const skip = config.command !== 'build' || config.build.ssr

...

    load(id) {
      if (id === resolvedModulePreloadPolyfillId) {
        if (skip) {
          return ''
        }
        if (!polyfillString) {
          polyfillString = `${isModernFlag}&&(${polyfill.toString()}());`
        }
        return { code: polyfillString, moduleSideEffects: true }
      }
    },