我想让es6到我的项目，所以我用这个教程this tutorial，当我试图写'webpack'在cmd我得到的错误

Done in 8.99s.
{ Error: Cannot find module 'webpack-cli'
    at Function.Module._resolveFilename (module.js:470:15)
    at Function.Module._load (module.js:418:25)
    at Module.require (module.js:498:17)
    at require (internal/module.js:20:19)
    at runCommand.then.result (C:\Users\adirz\AppData\Roaming\npm\node_modules\webpack\bin\webpack.js:62:14)
    at process._tickCallback (internal/process/next_tick.js:109:7) code: 'MODULE_NOT_FOUND' }

package.json

{
  "name": "functions",
  "scripts": {
    "build": "tsc",
    "serve": "npm run build && firebase serve --only functions",
    "shell": "npm run build && firebase functions:shell",
    "start": "npm run shell",
    "deploy": "firebase deploy --only functions",
    "logs": "firebase functions:log"
  },
  "main": "lib/index.js",
  "dependencies": {
    "firebase-admin": "~5.12.0",
    "firebase-functions": "^1.0.1"
  },
  "devDependencies": {
    "ts-loader": "^4.2.0",
    "typescript": "^2.5.3",
    "webpack": "^4.5.0",
    "webpack-cli": "^2.0.14",
    "webpack-node-externals": "^1.7.2"
  },
  "private": true
}

我的文件夹结构