vue-cli/packages/@vue/cli-service/lib/PluginAPI.js

const path = require('path')

module.exports = class PluginAPI {
  constructor (id, service) {
    this.id = id
    this.service = service
  }

  resolve (_path) {
    return path.resolve(this.service.context, _path)
  }

  setEnv (env) {
    process.env.NODE_ENV = env
    process.env.BABEL_ENV = env === 'production' ? env : 'development'
  }

  registerCommand (name, fn) {
    this.service.commands[name] = fn
  }

  chainWebpack (fn) {
    this.service.webpackChainFns.push(fn)
  }

  configureWebpack (fn) {
    this.service.webpackRawConfigFns.push(fn)
  }

  resolveWebpackConfig () {
    return this.service.resolveWebpackConfig()
  }
}