vue2/packages/vue-server-renderer/index.js

try {
  var vueVersion = require('vue').version
} catch (e) {}

var packageName = require('./package.json').name
var packageVersion = require('./package.json').version
if (vueVersion && vueVersion !== packageVersion) {
  throw new Error(
    '\n\nVue packages version mismatch:\n\n' +
    '- vue@' + vueVersion + '\n' +
    '- ' + packageName + '@' + packageVersion + '\n\n' +
    'This may cause things to work incorrectly. Make sure to use the same version for both.\n'
  )
}

if (process.env.NODE_ENV === 'production') {
  module.exports = require('./build.prod.js')
} else {
  module.exports = require('./build.dev.js')
}